WordNet (r) 3.0 (2006):

sex act
    n 1: the act of sexual procreation between a man and a woman;
         the man's penis is inserted into the woman's vagina and
         excited until orgasm and ejaculation occur [syn: sexual
         intercourse, intercourse, sex act, copulation,
         coitus, coition, sexual congress, congress, sexual
         relation, relation, carnal knowledge]

Moby Thesaurus II by Grady Ward, 1.0:

44 Moby Thesaurus words for "sex act":
   act of love, adultery, aphrodisia, ass, balling, carnal knowledge,
   climax, cohabitation, coition, coitus, coitus interruptus,
   commerce, congress, connection, copula, copulation, coupling,
   diddling, fornication, intercourse, intimacy, lovemaking,
   making it with, marital relations, marriage act, mating, meat,
   onanism, orgasm, ovum, pareunia, procreation, relations, screwing,
   sex, sexual climax, sexual commerce, sexual congress,
   sexual intercourse, sexual relations, sexual union, sleeping with,
   sperm, venery