Mine is on the pill, and I always pull out. I've had girlfriends in the past who were on the pill that I came in all the time and never got prego. I had a scare earlier last year where an ex of mine that I was sleeping with at the time who was on the pill and had polycystic ovarian disease(this apparently makes it that much more unlikely for her to get pregnant) found out she was pregnant, but it ended up not being mine. Since then I don't think I've cum inside a girl. Actually maybe once when my current gf was on the rag. It's just not worth it, even though it feels really really really nice.
If I remember correctly, birth control pills have a 99% rate of effectiveness. There is always that small chance that you're going to get her pregnant. And from what I've heard, cumming inside a girl is a really good way to get her pregnant. Some might venture to say that it's the preferred way for couples to conceive. I personally prefer the method where you spend an entire year jacking off into a Folger's coffee can. Then after a year you stick a funnell on the end of a hose leading into in your breeding mate's cha cha and pour the contents of the can into the funnell while shouting "GO GO GO!!" with several of your buddies hanging around to watch.