I am going against the grain here and will say YANBU @Kala101
It would not sit well with me either.
Sperm donors USED to be able to remain anonymous, but not anymore, and yes, you COULD get a tap tap tap on the door in 15 years or so (depending on when he donated his tadpoles,) from someone wanting to meet their 'daddy.'
It put many men off donating when the anonymity was taken away (unsurprisingly!) But some men still do it.
Some of what you say is a bit odd though, like 'ewww, my boyfriend's sperm is in these women!' But maybe you just worded what you wanted to say, a bit badly.
But yeah YANBU. This is not like donating blood, or bone marrow, or food (to a food bank,) or even a kidney! He is basically donating human lives! People. Actual offspring of his.
I don't know why you are getting such a hard time. I would find this hard to accept too.
I do agree that you need to split with him though, as this is never going away. He actually sounds too blasé and devil-may-care to me, and seems to care about himself more than you. He doesn't seem to care about your feelings at all. You deserve better. End it now.