In response to your actual question - I expect I would be somewhere between indifferent, proud, and (slightly) intrigued. I hope I would support my partner to build an appropriate (but non-parental) relationship with the adult child, and I’d expect to also be part of that relationship. I think the more fitting comparison would be to an adult DC’s partner, rather than to a step-child - it’s not a parental relationship, it’s not one I’d have the right to insist didn’t happen.
I have sympathy for everyone in this situation though. Definitely it’s a huge shock to the wife. It’s also a shock to the donor, who would have never expected this to happen way back when he decided to donate. And it’s a whole bunch of feelings for you.
I have donor-conceived children, who know their donor; and somewhere out there is a small child who was made possible by me donating my eggs to a couple I didn’t know. I’m delighted I could help her parents have a much-wanted baby, and if she comes looking for me later down the line I would definitely be interested in meeting her and happy to fill in any questions she has, but she’s (obviously) not my child and tbh I go months at a time forgetting all about her - and I know she was born, and I signed up to the current (IMO hugely preferable) arrangements for ID-release donation. So I can also imagine that a ‘may well not result in anything at all’ donation, from years and years ago when loads of guys were doing it and when there was no expectation that they would be traceable, would just slip his mind and not seem worthy of mention, although I guess I’d expect it to come up in conversation at some point over a lengthy marriage (when there’s been a TV programme or someone else you know has conceived via gamete donation or whatever), and perhaps the absence of that mention when the chance must have been there must feel a bit deceitful. It’s never occurred to me that I should tell prospective partners very early on about my donation, but I guess since I also have actual children of my own that’s rather different.
I think lots of this thread is poorly informed (eg all the fretting about wills), and I think also depends on whether one essentially views a donor as a parent or not. Some people clearly do (I think mostly not when they have had direct involvement).
I agree with PP that the dramatic ‘this is a dealbreaker! I would never have entered into such a marriage if I had known!’ response is akin to wanting only untainted virgin spouses. Which I suppose is perfectly reasonable, but I don’t get it and I don’t much respect it either.
I’d echo P999’s suggestion of making use of the HFEA, and I hope this thread isn’t making what’s already an emotionally tumultuous time for you any more difficult.