Why at the final moment girls say "We shouldn't be doing this..."
Maybe you're about to embark on a boat-tossing make-out voyage. Maybe you're about to dive headlong into the depths of her stormy sea. Maybe you're only chatting harmlessly at the bar. But at some point, she halts the progress with "I don't know... we shouldn't be doing this..." I'm not talking about when she says, "OK we're done," where she really does want to stop. She obviously wants to keep going in the "we shouldn't be doing this" case, so why does she say this?
One standard answer is that it's a disclaimer that she can use if she should have to suffer the negative consequences of doing something she knows she shouldn't do. For example, she might get labeled a slut, so this phrase gives her plausible deniability -- "Hey, I said we shouldn't be doing this, and I knew it was wrong, so don't judge me so harshly." This is the cover-your-ass theory.
A second standard answer is that she is probing some individual quality that you have, like how strong or manly you are -- the more masculine and dominant you are, the more likely you are to brush her disclaimer aside. The assumption is that girls prefer sleeping with strong, dominant types and have to figure this information out some way or other. This is the test-your-manhood theory.
Both of these theories are plausible accounts of just this one test, but we gain more insight by trying to explain a host of related cases with the same small number of explanations, not by tossing out ad hoc guesses after seeing each case. Here are some related cases that cast doubt on both of the above theories. They all involve one person (the actor) acting in a way to further cement a social bond, and the person or persons on the other end of that bond (the beneficiaries) exhorting the actor not to do whatever he's about to do.
1. A group of close friends are hanging out and talking together in a bar, when some slimeball comes up to the girls in the group and makes a variety of lewd remarks and tries to fondle them, although never getting violent with them. The girls give him looks of disgust and try to slap away his hands and push him back, and he walks off. Everyone in the group starts talking about how awful that was, all agree that he deserves to get the shit beaten out of him, and that he'd be better off dead. At the height of this discussion, one guy (the actor) announces that he's going to go over to the creep and beat him up. The girls implore him not to be so silly, you shouldn't do that, etc., but they do not put their foot down, like "No, you'll stay right here young man."
Everyone in the group knows that it would violate some code to go after the creep long after the fact, rather than notify the bouncer or security and have him removed from the club or something. So when the girls say, "You shouldn't do that," both the actor and beneficiaries are aware of that. But since the girls are not firm, it's clear that they want the actor to go over and kick the creeper's ass. (Similarly, a grenade gets thrown into a platoon's area, and one brave soldier declares he's going to run and cover it, while the others beg him not to, although they don't do anything to restrain him.)
The cover-your-ass theory fails here because the girls cannot be held responsible if the actor beats the creep up. And the test-your-manhood theory does not work because, unlike how they choose their sex partners, girls do not choose their friends based on how macho they appear, so they are not constantly trying to figure out if you're as manly as you claim.
2. Two close friends agree to go parachuting together, and right before the first is about to jump out, the second one stops him to say, "We must be crazy... you know we really shouldn't be doing this." Again the tone is not sarcastic or humorous, nor is he firm. His tone suggests he does want both of them to jump out. Similarly, the two might be about to go on a joyride that they've been getting excited about after talking about how fun it would be, with the friend in the passenger seat saying "We shouldn't be doing this." Both are aware that the act violates what they take to be their normal level of risk by a longshot.
The cover-your-ass theory fails again because the second one cannot be held responsible for anything that happens to the first one after jumping. The test-your-manhood theory fails for the same reason as before: guys don't choose their friends based on how macho they are (they may even want someone not so macho, to minimize dominance contests).
3. Two close friends are threatened by a common enemy who they would rather not stay and fight. They'd rather run away from the problem, but their escape vehicle requires two people to operate it (like a pilot and co-pilot). Just as the first friend hops in the pilot's seat, the second one says, "This isn't right, we shouldn't be doing this..." Again the tone of voice indicates that he's not firm and wants the plan to continue. Both are aware that fleeing violates some code against facing your enemies, or at least taking the enemy out lest it turn on others in the community once the friends have fled.
The cover-your-ass theory somewhat works here. If they are found out as cowards, the friend who expressed doubt may get more lenient punishment than the one who said "full steam ahead." The test-your-manhood theory fails here because brushing aside the disclaimer and carrying out the plan to flee suggests that he's a coward, not a macho man.
There are many examples like this, but overall we see that the two theories do not do very well. Rather, the reason that the objector raises the "We shouldn't be doing this" issue is that they want to see how committed and faithful or loyal the actor is to their relationship. If the actor replies, "I know we shouldn't be doing this, but I just can't help it," then he proves that his actions on behalf of furthering and deepening the relationship are not under his voluntary control. If he doesn't get into the relationship for rational, voluntary reasons, then he cannot bail out for rational, voluntary reasons. Showing that your commitment to the relationship is not under voluntary control is a credible signal of your trustworthiness, something that all people are worried about regarding their allies and mates.
In case 1, the girls are waiting to see if their guy friend is devoted enough to their friendship that he'll risk injury to himself to avenge their honor. If he says, "Yeah y'know what, I should probably just stay here," they'll feel betrayed -- or at least let down, believing that he's more of a fair-weather friend.
In case 2, the objecting friend is waiting to see if his buddy is devoted enough to their friendship that he'll take what he knows to be an unusually great risk just to say, "We went through that shit together." If he says, "Yeah y'know what, I should probably just stay here in the plane and ask the pilot to land," the objecting friend will feel betrayed or that he has only a fair-weather friend.
In case 3, we see the same as case 2. The objecting friend wants to make sure that the other one is committed to fleeing in cowardice together, lest the objector be left to flee alone while the first one goes back to take on the enemy.
And in the case of the girl saying "We shouldn't be doing this" before something that may in some sense be wrong, she's testing not your manhood but your commitment to her. You wouldn't risk getting in trouble over a girl you didn't care anything about. Suppose you could get fired for sleeping with one of your students or co-workers. After hearing "We shouldn't be doing this," you wouldn't dare say "I know, but I can't help it" unless you really were stuck on her. If you merely wanted to proceed because you wanted to bed her, you'd say something more logical like, "Don't be silly, they won't find out" or "It won't be a big deal if they find out" or something else that tries to understate the severity of the risk. By saying, "I know we shouldn't, but I just can't help myself when I'm around you," you show that you're willing to pay a high social cost to be with her. The costliness of this act makes it an honest signal of your trustworthiness -- at least that you can be trusted with her, even if not necessarily with others.
Of course, if she was not interested in the guy in the first place, his costly signal of devotion would only creep her out more -- like John Hinckley, Jr. trying to shoot Reagan so that Jodie Foster would notice him. Remember, we're talking about a case where the guy and girl are already about to do something big together, where basic trust and familiarity have already been established. So instead of getting weirded out, she thinks, "God, he's willing to sacrifice his job for us -- he really does care." Remember also that we're talking about where her tone of voice is not firm but shows she wants it to happen. Thus, she won't think he's being intolerably foolish to risk his job; otherwise she would put her foot down and not allow him to put himself in harm's way.
In our cynical age, people are too eager to wittily toss off ad hoc explanations of human behavior where our motives are variations on petty status-seeking, like covering your ass and looking more macho than other guys. Once we open our view up to a fuller range of similar cases, it looks like the explanation has to do with the same small number of Big Themes of the human condition -- love, trust, betrayal, revenge, etc. That is surely why Shakespeare, Marlowe, Kyd, et al. continue to be read today, while overly cynical and witty authors are mostly forgotten -- with the latter, not only is their writing more boring, but they don't even seem to get things right.