The masochistic fetish wouldnt be enough to keep the population going, because you need libido for masochism to work. Masochists want pain, but accompanied by an orgasm and the regular pleasure of copulating along with it.
Probably about 30% of the babies born today would still be born. Yes, I pulled that statistic out my ass...but it sounds somewhere about right.- of the approm. amount of people who would want to have kids enough to have kids no matter of making them was enjoyable or not. A good deal of those ppl would still have kids even if something painful or horrible was required to make them instead of fun. I mean, it's painful to HAVE a kid and that doesn't stop people from having more than one.
Ahhh but you forget to remember.. in the heat of the moment when another person turns you on and wants to take your clothes off and stuff you get all horny and it takes away inhibitions. Thats one reason why so many babies are born people just go "ok wheres the condoms!"... then its "oh, we dont got one." and instead of going downtown to get a condom there all "Oh FUCK IT!!" and they go right ahead and do their business. Then once someone cums there all "oh shit! we might be pregnant!".