the act

   copulation

   Sometimes tout court but more often as the act of shame (if outside marriage); of generation, of intercourse, of love; or the sexual act:

    My prepuce contracted so that the act would have been difficult. (F. Harris, 1925)

    ... she with Cassio hath the act of shame A thousand times committed. (Shakespeare, Othello)

    The embrace of the sexes in the act of generation. (EDD)

    An act of intercourse took place, in the course of which both partners achieved climax. (Amis, 1978)

    It was the time after the act of love. (M. West, 1979)

    The sexual act is fully covered, but not in these pages. (Longstreet, 1956)

   However, a sexual act may imply no more than a pinched bottom.