I think you are right, this isn't fair. I'd never be ok with that.
Everyonne should be free to decide individually what to do with their own body and its contents. The couple/marriage is just social convenience, not organic merge.
Anyway that's just theory, in practice I think the weakest link in the couple will eventually capitulate in this kind of delicate matters. Are you the weak link?
It sounds as if you are blessed with an incredibly caring, generous and giving wife. Perhaps you could focus on that and the child you two do have instead of one (or more) you might want.
The thing is she wouldn't be "having a baby with another man", even if she was the surrogate. She's having someone else's baby for them, because they're not able to.
To think otherwise, honestly, just kinda makes you a dike.
Well, I'm sorry that it comes off as such, but I think you're being rude to your wife and the other family by being so possessive about an egg cell.
It certainly isn't black and white, but you've painted it to be quite like that yourself when you claim that it's infidelity that your wife's egg cell be joined with another man's sperm cell. Whether or not someone is rude does not diminish the value of their actual argument.
Why should they adopt when they've been offered a chance to have a child that they are biologically linked to?
Edit: At the end of the day the way to approach the situation is to pretend that the egg your wife is donating is not hers. Just pretend it's the other family's, she's just renting her uterus for them. If this still bothers you then I don't know what to tell you, because you seem to have some serious trust issues with your wife.
You should be talking to your wife about this, not asking random people on a gaming forum.
So?
Imagine you were in their situation, your wife couldn't have children, some other family has offered you this opportunity, would you take it? And then you say yes, and then it gets called off because the husband in the other family got into a tiff because he thought artificial insemination counted as infidelity.
Put yourself in someone else's shoes for once.