Adults chastise adolescents for being too focused on the short term when playing the dating and mating game, but I wonder whether we're really so neglectful of cultivating longer-term prospects for marriage. Perhaps that's what mixed-sex friendships are when you're young -- an investment that only pays off later in life, when you're looking for stability and a soulmate.
Now, it is true that when you look back on all the girlfriends, flings, and crushes you've ever had, the grown-ups were right: they were mostly based on fleeting excitement. Even so-called committed relationships don't last a lifetime, or even close. Here again it's exceptional if they last beyond the endorphin rush of the honeymoon phase. Just as we wouldn't call someone monogamous who was married to one woman at a time but went through 5 wives, a kid who graduates college with 5 "serious" relationships since puberty has not been truly committed.
Kept going by the thrill of novelty rather than cemented by an emotional bond, these relationships are unlikely to leave either person reflecting years later that "that was the one that got away." Instead, you either feel lukewarm or ask yourself "What was I thinking?" That is, unless you already started out as friends before moving up to boyfriend and girlfriend.
I can think of only three girls who I would marry for sure (and a few other maybes), and they were all friends at various points throughout adolescence -- my first good chick friend in fifth grade, my close friend from eighth through twelfth grade, and a floormate from my freshman dorm at college, our first year of living away from our parents.
What they share with each other, and what distinguishes them from other girls then or since, is that we socially transformed close to each other, helping the other through it. That creates a bond of trust and fellow-feeling that does not arise when people get to know each other in more mundane circumstances. I haven't seen or spoken to them for 8, 11, and 19 years, but it's still there.
I'll elaborate on that theory sometime later, and show some vignettes of those three friendships that will hopefully make it clear why they're so different.
To end on for now, I know you don't have to feel this emotionally bound to each other for a marriage to work out. You can always remind yourself consciously that breaking it up would violate this or that norm, would have this or that bad effect on the children, or what have you. Still, relying on conscious reflection upon abstract rules is asking too much of most people. It's just not how our mind works in general. It would be better if we felt so attached that the temptation to cheat, leave, etc., didn't tug at us very hard in the first place.