I have seen this work well, and fail. In the case it worked well, the donor already had a family of his own, and was happy to be supportive and loving towards his donor son, without too much influence.

In the case where it all went to hell, so to speak, the donor was a lovely and caring gay male, with no children of his own. He came to see the donor son as 'his son' and became desperately attached, despite initially thinking that this would never be an issue. It was very sad for all involved, ending in the courts. He now has shared custody and it's a pretty hostile and toxic environment whenever all three 'parents' are together...

buliej made some really good points. At least, I would be seeking some legal advice...

There are some parenting decisions that people can feel REALLY strongly about, and it's amazing how much tempers and passions can suddenly flare up...