It only happens when the guy has the right motivation to protect them, and especially when they reward him for being there.
Tonight at the 16 - 18 dance club, a large group of girls I was standing near were repeatedly bothered by some 5'2, doesn't-understand-no, flabby spic. * He's the guy who breaks into a group he doesn't know, acts high-energy and goofy, and after having his foot in the door, proceeds to get too friendly with a girl (usually only good-looking guys can get away with a cold approach this fast and direct). She tells him to buzz off, and he gets upset and returns to do it all over again, each time talking trash to her in revenge. I'd seen him acting retarded all night throughout the club, and figured the girls he was approaching were simply amused by his goofiness.
But about 45 minutes before the club closed, there weren't very many girls left to harass, and a decent fraction of them were in one group near me, so I got to see how pissed off they were at him. Most 16 to 18 year-old girls are too immature to protect themselves in most situations, and a loser male can talk their ear off -- unlike the experienced 26 to 28 year-old women who would quickly cut such a guy down to size. This degenerate fatass was so irritating, though, that the girls began getting in his face, pushing him away, giving him the "don't come near me" hand, etc. You know it's gotten out of hand when girls start to stand up for themselves.
I'd had my fun for the night, I wasn't feeling as chatty or social as before, for want of alcohol, so I figured why not get some practice shoving another guy off of my territory. Any time he came near the group, I moved around dancing so that I blocked his path. Sometimes he would sit down next to a poor girl who had an empty space to her right, so if I saw him head toward her, I'd sit beside her first. He held a strong grudge against one girl who was the center of attention for her gymnastic and dancing skills, and any time he moved toward her, I moved right away to stand in front of him, while still dancing or checking text messages. I never lost my cool, always had an amused smile on my face, and was not overtly confrontational in any way.
I was not afraid, though I thought he might throw a punch at some point because he was wound up the whole night. Only a loser would act so desperately and foolishly, so nothing to fear. A loser will not stand up to someone they perceive as far above them. It didn't hurt that I was wearing a shirt, tie, and sporcoat: you never know when you'll have to put someone in their place, and dressing in a powerful way makes it go much more smoothly. He got in my face once or twice, and even tried to AMOG me by giving me dance tips (only works if you're a better dancer than I am, and not some bloated wetback oaf). Eventually he got the hint and came back less frequently, then stormed out of the club altogether.
Aside from the intrinsic pleasure I get from pushing losers around, I felt compelled to protect the girls, even though they weren't my friends. We all have our weak moments when our sense of duty and responsibility jumps up for a bit, and triggering these weak moments is far easier for a 16 year-old girl than a 26 year-old, again, because the former is too inexperienced to know what to do, and it's clear that someone has to step in.
Those who wish males would assume more traditional manly roles like protector of strangers must bear in mind this inherent constraint: some strangers are better motivators than others. It's not as bad as it sounds, though, since those who don't easily motivate guys to protect them -- other guys and older women -- tend not to get into such sticky situations (older women) or can expect to fend for themselves (other guys).
Although I didn't expect anything in return, they got pretty turned on. The girl on the end, who the guy kept sitting next to, began stroking her hair (but she was blonde, so I didn't move in on her). The gymnast / dancer cutie, on the other hand, got quite a hard-on: exhibitionistic gymnast types will only take a shine to guys who display honest signals of machismo. Suddenly, she conspired to dance into my personal space so that our arms and thighs brushed, our hips checked each others, and her ass found my hand. She did this between 7 to 10 times. The whole time, she had that cocky grin that bad girls get when they're pursuing an intriguing novelty (like a guy 10 years older than her).
By this time, the night was almost over, so I didn't want to rush an approach. I see her there every weekend, so this will provide a good opener for when I see her shakin' her thang next Friday. She had an even hotter friend, who also took notice of my protective efforts, so maybe I can get her to introduce me to her friend. She's Black, by the way -- only rarely does a Black cutie seek out a White boy who hasn't even signaled his interest in her first. Even more strangely, she had medium brown skin and slightly Eurasian features (her hotter friend was also Black and had even lighter skin and more Eurasian features) -- usually these girls only want to be seen with jet-black gangbangers who assuage their worries about not appearing Black enough.
So who says no good deed goes unpunished? In too many instances, the "good deeds" that men do require nearly no effort or sacrifice and are really just complying with simple norms of politeness. You only get rewarded for your good deeds when you risk something in doing so, and especially when you help out someone who can't help themselves.
* As Chris Rock might distinguish them from Mexican-American people.