There's your mistake. There is a difference between "wanting to" and actaully doing so. I've *wanted* to have sex with several women but I've always resisted the temptation and am still single. Maybe she would have said no and maybe I didn't want to have to deal with the rejection or perhaps I would have left her feeling insulted that all I really wanted was sex when I actually wanted more.
You're worldview is the common one where sex *is* the endgame for men. I'm not that way and never have been. I want to give and receive love, not just have sex but it's become difficult to find even women who grasp that concept.
Probably the best thing to do is to resist up to a point. If you really do love the woman, you've been dating for a while and she is good marriage material, you should take the plunge (Pun not intended, but interesing) and risk it. If she gets pregnant, well, you already love her, so why not commit for a lifetime? Your chances are still good and she'll give you children. And besides, why carry romantic notions that passionate love is for a lifetime? That's the stuff of fairy tales. After the honeymoon faze, it's normal for things to calm down. The important thing is to find someone to commit to, and for her to commit to you.