A fifty cent word for sex that, while socially acceptable, also carries a bad connotation, suggesting that the action is animal-like and instinctive, having no real basis in human emotion. It's a way of taking what might be seen as making love and turning it into a simple action without resorting to terms like fuck that might bring up images of lust and passion.

Cop`u*la"tion (?), n. [L. copulatio: cf. F. copulation.]

1.

The act of coupling or joining; union; conjunction.

Wit, you know, is the unexpected copulation of ideas. Johnson.

2.

The coming together of male and female in the act of generation; sexual union; coition.

© Webster 1913.