OP This reminds me a lot of the brilliant old Mike Leigh film Secrets & Lies if you have never seen it you should.
You do need to find great compassion for the wife. She is not the wicked stepmum here. She has been wronged by your father's secrecy and now the surprise of you. Why wouldn't he mention during their marriage he was a donor,? He chose to do it, to keep it silent, and now is considering a relationship with you. His wife is robbed of all choice.
Then they tore up the donation contract which is so wrong someone should have taken them to EU court for that so anonymous isn't anymore, and he didn't get a vote on that. He's also helped countless families too so his 'karma' isn't all bad but his wife now has to deal with his secrecy, the shock of a new adult child/ family, and the real terror that more, perhaps many more children might turn up and frankly have claim over something they built.. and on him.
I agree with a poster here that his secrecy + reaction to you is suspicious. This could get very nasty quickly given what's at stake.
The way I see it everyone is a victim on some level here. I cannot imagine the sense of shock and confusion for you, your father, and very much for his wife too..