If you don't mind me asking, does it raise different issues, or are there different background assumptions, in considering whether to be monogamous when dating another male?

Well, I've never dated a female, so I can't actually compare the two. I wouldn't expect there to be different issues and assumptions because we're a gay couple. (Apart from the obvious stuff like family disapproving, of course.)

I should note, however, that this is a back-door relationship: it started with us talking about sex in general, then it progressed to talking about sex with each other, then it progressed to us feeling jealous at the thought of each other having sex with anyone else, at which point we decided to consider ourselves in a relationship.