But it's a greater than zero chance that it will end in pregnancy. Actually, it's about 1 in 4 that it will end in pregnancy.
Killing babies isn't a solution for anything, I don't care if it cured cancer, parkinson's and AIDS all in one swipe just by the act of killing, it's not worth the cost ethically, morally, or spiritually.
On the point of social sex, sex is a pleasureable(or so I've been told), bonding event because it helps a mated pair maintain the relationship in a positive way. Despite what you think, until very very recently, casual sex resulted in the spread of some rather horrific diseases so there was a definate social downside to leading a loose lifestyle.