I'm going to come at it from a likely unpopular perspective for you to try to help:
Look, if you do have an underlying issue with it that perhaps your son has picked up on, which I do suspect as do others here, it's okay you know. You can't help feeling a certain way.
But, if you do, there is literally not a thing you can do about it. So, treat it as though he was with a woman. It's not that it doesn't matter, of course it has implications to childbearing and social attitudes, and it's okay to have thoughts about these things.
But, there is nothing you can do about it. The only outcome you could possibly achieve by expressing to him that you view his relationship differently to if he had chosen a female, is to alienate him.
If you don't want to alienate him then unironically give into the fact he is having a relationship with a man. Be pleased for him, genuinely, because it is the path he wants to follow. Just literally don't give a shit because there is no point in giving a shit.
Let it happen. Ask to meet his partner, cook his partner's favourite meal and get to know him as a person as he is probably lovely. Accept him and let him come over and be part of your family.