Single, never married and in my mid 20's.
While I was never married, I did have one very long term relationship which came very close to marriage at one point; we were living together and had been discussing having kids.
Similar to the stories many of those of you who are/were married, sex life went to hell, she got extremely fat, and generally the love in the relationship had gone. She had cheated on me before when the relationship was still good, and I never really got over it properly, which then festered into resentment. Different to the people here, it was actually me who was refusing sex; she was hugely overweight, and I resented her for her previous infidelity as well as her general treatment of me, which was pretty vile. Not sure why I stayed in the relationship; I think because generally I used to feel like quitting was a terrible thing, and so I would try and struggle through, making compromises, trying to understand her, get on her good side, etc. It's easy to see from the outside what a disaster it was, but it had just taken so long to get to that point, and used to be at such a high, that at the time I just couldn't see how bad things were.
Anyway I finally pulled the plug when I discovered she had been shagging most of the country from near enough the start of the relationship; even during the really good times when I had thought we were madly in love. I was absolutely devastated, it took me a long time to get over it, and it was really what prompted me into punting.
In hindsight, in a way I'm glad to have had the experience as I think I've learned and grown a hell of a lot from it. Hearing a lot of stories about people's marriages that went sour, it seems like the exact sort of thing I went through, only I've managed to escape it without the huge upheaval of a divorce. It's completely reshaped my view of marriage; where previously I thought of it as something everyone should seek out one day, and should stay together "till death do us part", I now just don't think it's viable. There are some couples I know who will probably have very good marriages, but I don't think it's for me. I posted in another thread a brief comment on how monogamy is perfectly natural, but I just don't think it's meant to be a lifelong commitment. In my view it's effectively a temporary pairing to raise a child, and shouldn't be the all consuming lifelong commitment that it is, as I just don't think it's meant to last that long.
I'm pretty firm on the idea that I'll never get married. Kids? Yes. Live with someone? Probably. Marriage? Never.
Perhaps that's not the ideal situation for the kids, but I don't think a child is any better off growing up in a loveless house with two parents who resent each other.