One problem: humans, even in western societies, aren't strictly monogamous. Men and women both can, and not infrequently do, have more than one partner, even when married.
Of course, another explanation is the men and women have sex for "their own pleasure" and "their own reasons". That somewhat begs the question of what those reasons are. But that could OK.
Sorting out social, biological and "personal" motivations is complicated and interesting problem.