...Not like that, but, let's say it happened. Greed would probably be utterly incapable of monogamy and would spend however much time it took convincing Issei that sleeping around is a GOOD thing. If he had to, he could probably hold out two to three months if he was serious about wanting to make Issei okay with polygamy (polyandry) first, but after that he'd have to break down and call it off. At least he's honest, so he'd not sneak around about it. He can be the jealous type but it'd be hard to trigger, unless it seemed it was losing tangible ground or interest to someone.

I can't speak for how Issei would act, but if he's feeling manipulative, Greed responds strongly to blunt, straightforward statements like "but xyz hurts me and makes me sad." So, physically it might never happen, but Issei's probably sharp enough to be the emotional top easily, just as long as he doesn't try to interfere with Greed's line of work... which I suspect he might have issue with, but Greed's willing to never talk about it if Issei is! Unless they got into a big fight, in which case he'd play up the "you're screwing a murderous criminal thief and you know it, so get off your high horse!" card, but by that points they're surely already headed to a breakup.