My wife and I had a huge talk about this last week. She's always said that children outside of the marriage are a big no-no. (My intro post elsewhere goes into detail about how that has somewhat stifled a relationship I'm in as my friend is 39 and really wants a child soon.)
What started the conversation is that a couple we know are struggling to have kids and my wife offered to donate eggs. Without discussing it with me first! I know that's not quite the same as her having a baby with someone else but I wanted to know exactly where the line was. Could I donate sperm to a friend? Apparently not! The reason she gave was that if I did, I'd still be on the hook legally as a parent and that would involve time and financial responsibilities.
So we talked about those two things in depth. My friend is financially independent and really wouldn't need much help. She's also very proud and would refuse most help that was offered. So that shouldn't be an issue. As for the time thing, she's pretty determined to have a baby soon with or without a partner. I pointed out that if she did have a baby on her own I'd still be spending time with her and helping to raise the child and my wife said she would actually be ok with that as long as it wasn't my child.
So my wife has tried to rationalise it as best she can. But what it really seems to boil down to is that I'm hers and the one thing she won't share with anyone else is my DNA. That seems to be quite a low level instinct that no amount of logical debate will get past. It doesn't quite make sense, but it's what it is for us.