because it's hard to be sexually uninhibited if you are scared of getting pregnant.
the invention of the pill was, in my opinion, probably *the* most important factor in changing the lives of women in the 20th century. Fantastic, we can have sex for fun - you guys have taken it for granted!
condoms and caps are statistically fairly safe but when you are using them you feel very aware of the link between sex and pregnancy. Or I did anyway.
a sterile man offers the same freedom.