Thanks for your replies RunningWithScissors and nicole83 - it's so great to have words of wisdom from people who have gone through this. 48ish hours after finding out, we're doing ok. We know we still want to raise children together, and if we need a donor to do that then we'll come to terms with it. As DP pointed out, we have two fur children (cats) - they're two of the most precious things we have, we love having them in our lives, and they aren't even the same species as us. So many families have step-parents/kids these days, and no one thinks twice when the step-parents have a closer bond to the kids than a biological parent that doesn't have much to do with them. And in the case of a donor, not only have they 100% agreed to the existence of a child raised by someone else, there's also no access visits to worry about or arguing about child support - that's a better scenario than a lot of families I know!
We did have a moment last night when we realised that the family trees we'd traced recently might not be relevant to our offspring (we got back hundreds of years). That kinda hurt, and I suppose those little moments are going to keep coming for awhile. Still. We still have weeks before we know what we are up against, so for now we'll just wear loose underwear, keep taking menevit and elevit respectively, and see what happens!