He’s wrong, she’s a twat. He’s going off with her like a stupid puppy because he likes how he feels when he’s with her because she MAKES him feel wanted.
But, just playing devil’s advocate op, how do you MAKE him feel. Imagine you were gallivanting with another man and said “hey dh, I’m going to France with my new man friend tomorrow” and your dh just went to the loo and then sat on the sofa sulkily, how would you feel? And if he said he didn’t want you to go because it was inappropriate but then helped you pack and find your passport? Wouldn’t you feel like he wasn’t THAT bothered? Wouldn’t you feel like he only cared about the optics, rather than actually desiring you?
if a man was posting this saying “my wife is going away with someone and I’m annoyed because it is inappropriate but I don’t want to tell her not to go because she should just put me first even though I’m not emotionally forthcoming and she’s lonely and has no friends of her own since she moved to be with me and my kids who she kindly ferries around to sporting events…..I feel like everyone would be here saying to him well tell her love her, tell her you need her, and respect that she’s making such an effort to put herself out there going to events and meet ups, being a good step parent and trying to make a new life in your area to be with you.
the reason I say this is not because he isn’t being a tosser (whether knowingly or not) because he is. It is more that, as you find it hard to express desire for him or to be emotional rather than logical, is there a chance you’ve not made him feel particularly desired for a while now?
i totally get it is hard to be emotional and theoretically he should just not have his head turned “because you’re together”, but the reality is he needs to feel like you want him - otherwise he will have his head turned by someone who does.
If you do want him, now is the moment to screw his brains out and make him feel like holy shit she wants me - she’s mine and I’m hers. Not to push him into the arms of this woman and her exciting French adventure!