I was in the no kids camp for quite a while then I dated and married a woman with a daughter a few years later she was pregnant and I now have two daughters.
The bitch has been out of my life for quite a while now but those two girls are the best thing to ever happen to me.

To be honest though, if I decide or had decided firmly not to have kids I would just go ahead and commit to my decision and get a vasectomy and avoid any chance.