So there are many hypotheses as to why humans are biologically available for sex throughout their life and menstrual cycles (post menopause, during pregnancy, for example) while most other mammals have constraints on when they can mate. However, many scientists believe that this is because copulation in humans (and in bonobos, chimps) evolved as a means of pair bonding first, and reproduction second. The majority of mammals can only mate during certain times of the year for strictly reproductive purposes, but humans and other apes are free to engage in copulatory behaviors as a means of building and strengthening relationships during any time of year and throughout most of their life.
IS THAT NOT FASCINATING? IT MAKES ME WANT TO GO PAIR BOND RIGHT NOW!
Anyway, this TED Talk is quite interesting (be warned that it is a bit heterocisnormative) and it explains why what society says we should feel about sex doesn’t often align with what we may actually feel about sex.