Don't be so stupid. Men and women both enjoy sex but pregnancy involves a lot of extra physical work for the woman because it takes ~9 months to grow another human being (or more than 1) that's 5-10% of the woman's own body weight, not to mention that human infants are completely helpless for a long time after birth, in contrast to many other animals.

This is just a fact of biology before you look at any of the social contextual factors.