Why should I not be open with my sexual fantasies if I want to be in a committed relationship with someone who shares the same sexual interests?

I don't talk about sexual fantasies on the first date. Most of the time I don't say anything at all until I get to know them. These guys aren't one night stands, they've been people I'd date for a couple weeks and then after we have sex they tell me "they've got a lot going on right now and they're not ready for that kind of commitment."

I just don't get why they back out after sex. Why lead me on?