Copulate means to have sex. It is a cold, formal, biology-class word, not a bedroom word and not a joke. Scientists use it of animals; doctors may use it of people in a medical note. Friends do not. Though actually, the Latin underneath is milder than the English feels: it meant to join or couple two things. English narrowed it to this one joining. If you use it of people in ordinary talk, it can sound like a lab report — distant, and a little unkind. For animals, it is the normal textbook verb.