Is it OK for my man to keep his ex's dog while she's gone for the night?

Hell no I wouldn't be ok with it. For starters, I wouldn't be ok with him staying in contact with his ex. If they have kids, I could deal with that but their conversation better stay strictly to the kids, not personal. He doesn't need a personal relationship with his ex, she is his ex for a reason. And dog sitting is obviously personal. I wouldn't be ok with it, and especially after your fight. Honestly I'd probably have left him for that. Whether he was mad or not that was crossing the line. How I would handle it is telling him I am uncomfortable with the situation. I would tell him I think it's innapropriate that he keeps personal contact with his ex. And doing her favors makes me even more uncomfortable. I would ask him to please fix the situation, and end the relationsip with his ex. Don't yell, don't get mad. Rationally explain the way you feel and how you'd like things to be. If he wouldn't fix it, I would leave him. It's only been four months, it wouldn't be too hard to get over it and move on. That's way too early on in the relationship for such big issues. You say it's justified because he was mad, I say him saying that in a fight is bull sh*t, and more that I should have to deal with.

"Update: I dunno."

Indeed, you don't.

If I were you I wouldn't worry about this. If I had to trust someone with one of my pets it would be someone who knew the pet's needs and idosyncracies. That would be your man.

It's only a dog!

I am the ex wife. lol. Ex husband and I share the maltese. We just take turns having the dog (kind of like child custody except we set our own terms). We both really love the dog to death. I don't know about him but I do not have feelings for him. We barely talk and if we do, it's strictly business. I don't see what's the big deal about it? It's a small cute innocent dog. Maybe the heart of the issue lies somewhere else? I think you are insecure of yourself because he told you he wants to be with her. Even then, it's your boyfriend who is the problem and not the ex-wife or the dog. I think you are taking your frustration out on the dog and it's unhealthy not directly dealing with the real problem because it will drive you insane.