This coming from the guy who was so butt hurt over his ex fucking other people when they were in an open relationship....
I knew this was coming and I know how you would make your case on the topic before I even filled in any information about it so there's no point.
I'm not married or living together with a significant other, but you could just say marriage and a relationship are interchangeable for the sake of this discussion.
Assuming those two things 1) you are in relationship and 2) you are living together, and then assuming said person won't have sex with you, absolves you of looking for other avenues.
Assuming only one of those two things married but long-distance or living together but not yet married, I don't think you can just go traipsing around for ass and expect the other person not to care.

Logged
Pour the wine, hold the grind, quarter to nine, let's go.