I'm of the mind that it doesn't matter, really. If you're going to be a slut, and know this before the IC marriage, then you should probably let the other person know, but otherwise...

You're roleplaying. You're only responsible for your character, regardless. It isn't up to you how anyone else acts, so why should how you want to control your character be up to someone else? If I were hunting with someone day in and day out, and never seeing the IC spouse, then to me it makes sense IC to become a little enamored of the new interest.

I think it really only has a bearing if there's RL feelings mixed in with the RP feelings. I don't necessarily mean romantic either, just that your feelings toward the other player interfere in the characters' lives. For instance, maybe you want to cheat on your spouse, but you enjoy being the friend of the player behind the character, and you're afraid that if you cheated their character would, all in RP, stop communicating with you - effectively ending the friendship beyond some tells, perhaps.

Expecting someone to be faithful to you, when there are no romantic feelings involved IRL, is poor form, when you are never around to interact with. And frankly, it can be a great way to break the monotony of a character and introduce some new storylines.