That's easy Ron. For me personally, I got the snip so I could not be responsible for a "surprise I'm pregnant " phone call from the girlfriend. When I had the snip, I considered myself too old to sire kids. I have none, don't want none. I'm quite happy to accept children already born, just consider myself too old to take care of more.
A good mate of mine had to "get married", because SHE decided it was time to have kids. He was not consulted in this decision. 3 months prior to this we were both chasing the same woman.
So when she got pregnant without his blessing, me and my mate came to the conclusion that she was deciding which one of us was to be the father.
Fortunately for me, I lost.
I got snipped a week later, and have never regretted it. That was 10 years ago. Different for other people I know, but it suits me fine.
By the way, I love kids and am not scared off by any future girlfriend having kids. I just believe in being around for them when they are growing up, and being fit enough to kick a ball around with them during those important stages in their youth.