I’ve known for years that i didn’t want kids, although for quite a while it wasn’t a particularly strong objection, because i was young and the issue didn’t really come up.
I only found out how strongly i felt about it when i was about 28 and had been going out with a woman for a couple of years, and we started talking about long-term plans for marriage, etc. I knew she wanted kids, but initially i thought that i could have kids if it meant being with her, and i told her that.
Then, after thinking about it for a few months (we still weren’t living together or anything), i decided that, in fact, not having kids was a deal-breaker for me, and i really didn’t want them with her or with anyone else. I told her this, and we broke up.
I was pretty conflicted about the whole thing for a while, because i really thought we were going to be together permanently, but in the long run we both made the right decision. She’s now happily married with two kids, and i’m married to someone who, like me, is happy without kids. I’m still friends with the original woman, although we live in different countries and see each other very rarely.