Alas, one cannot just talk about sex in human terms without having love mentioned. Chemical love is a big part in human bonding that affects many individuals to have sex they would not ordinarily have. You do not have to call it love but whatever you do call it the chemical affects on human behaviour certainly can and do induce intercourse in the species. Receptivity, orgasm, the actual chance of getting pregnant, are all significantly increased when those chemicals are present.
Anyways. Your thread. You choose the rules. I'll shush :-)