Maybe they enjoy the thrill. Maybe they're unfaithful by nature. Maybe they feel entitled to have multiple lovers.
Because a lot of people cannot control their innate greed. Or maybe they don't make as much as they'd like to (which, sort of, ties in with the aforementioned point). Or maybe they're just kleptomaniacs.
Which presupposes that there are no rational answers to these questions when, in fact, there are. It's much simpler than you make it out to be.
So, can we return, yet again, to the question of homosexuality as a choice?