I've said it before, and I'll say it again. Bad sex can kill a marriage.
By no means was sex the only problem in my marriage, but it was a big part of it, and kind of the beginning of the end. My wife had practically no sex drive. We started off with a bang, we had sex a whole ONE TIME on the honeymoon. After that we settled into a nice routine of once every week or two. When we did, it was barely worth the time or effort. Nothing but missionary. She seemed to prefer it if I kept quiet and finished as quickly as possible. Not that we could ever even discuss it, she didn't like to talk about sex. Terms such as "weird" and "gross" started being used when we did talk about it.
After 5 years of that bullsh!t (and the crumbling of the rest of our relationship) I decided I'd had enough, and started looking at what else was available. We're now in the process of divorce, and while it saddens me that it had to end up this way, I'm really looking forward to a future with a woman I can enjoy instead of one that I despise. I feel bad for her, she's been slapped pretty hard by reality (she was living a pretty charmed life while we were together) but she did bring it on herself. I gave her everything she could have wanted: a new house, a dog, a baby, she didn't have a job. And she couldn't bring herself to have sex with her husband more than every couple weeks.
Call me cruel. Call me an @sshole. I'm sure some of the self-righteous among you will. But in my position you'd have done the same thing.
Edit: For the waiting-for-marriage prone here, my wife and I didn't wait for marriage, but she was my first. I totally regret that. Had I been with other women, I don't think I'd have married her.