Just to put something in perspective here. People believe in romance because, as human beings, we are wired to hate being alone. Better survivability that way, you see. If you have a heart attack in your home and you live by yourself, game's over. You're dead. Chances of surviving go up when you're with someone else. But the fact of the matter is that who that person is doesn't matter. You may think it's about compatibility, but it isn't. Look at it this way. Suppose it was. Naturally you want to choose the most compatible person to marry, because why settle if you're more compatible with someone else? And that's the trap. Because consider everyone you know or have ever met and conversed with to a slight degree. Consider everyone you have on facebook. For this example, I'll use a fairly popular female friend of mine who has 600 friends. Assume, for the sake of simplicity, that you have a 50/50 split in terms of gender. That's 300 people in a perspective pool of people you know.
The kicker? Compared to the world population of almost 7 billion, that's not even a drop in the bucket. For the sake of simplicity divide that number in half for gender difference. 300/3500000000 = 8.57142857×10^(-8), or .00000008571% chance that the person most compatible with you is someone you know. You're looking for 1/3500000000. Your chances of winning the Texas Lottary are 1/175711536. You're twice as likely to win the Texas Lottary as you are to pick the human being alive, right now, who is most compatible. Oh, and if you're bisexual than you're four times as likely to win the Texas Lottary.
There are, of course, other complicating factors. Does that person share your sexual preference, for example. But those don't really matter. Point is, from that small pool of people you know the odds of finding your most compatible match is basically non-existent. And if you do, there's no guarantee things will stay that way. People evolve and adapt. Who you were once most compatible with might not be your match years down the road when you've become someone else.
And that's the simple math behind the illusion. Who you're with doesn't matter. It's just a fear of being alone and a desire for emotional gratification. Seems like a pretty good excuse for promiscuity now, doesn't it?