I assume that sex happened during the commercial break and that we didn't get to see it.

There is one episode in particular, can't remember if it was that insane lady who was trying to kill Kirk because his toupee offended her artistic sensibilities in the Shakespear episode, or another episode where we see Kirk and a lady in his quarters before the commercial break and then see them again after, with him on the bed, one leg on the floor. Sex definitely happened there.