Gestation is typically a function of body size, not because pregnancy is hard.  Animals don't time their pregnancies to make life easier, humans excepted, and that is generally a function of economics, not difficulty (witness Catholic families of the past or inner city moms).  

Sex and sex drive exist for one reason: to create offspring.  Because it feels good, psychologists like to find "scientific" reasons to make sex without reproductive intent something more than just guys being horny but it's not.  Many marriages survive for decades without sex after the kids are born.