Wordnet 3.0
NOUN (1)
1. 
 the act of pairing a male and female for reproductive purposes; 
- Example: "the casual couplings of adolescents"- Example: "the mating of some species occurs only in the spring"[syn: coupling, mating, pairing, conjugation, union, sexual union]
WordNet (r) 3.0 (2006):
sexual union
    n 1: the act of pairing a male and female for reproductive
         purposes; "the casual couplings of adolescents"; "the
         mating of some species occurs only in the spring" [syn:
         coupling, mating, pairing, conjugation, union,
         sexual union]