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A large percentage of gays don't even know they are gay, so they will think they are telling the truth.

They're also not going to be that scared, as there is no way to prove he knew he was gay when he donated sperm. If he has been seen having relations with a man previously, he could say he brainwahsed himself back into being straight with one of those homophobic, fundie sites. If he is seen having relations with a man, after the "donations" he could say he didn't know he was really gay at the time.

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"The state is the great fiction by which everybody seeks to live at the expense of everybody else... Everyone wants to live at the expense of the state. They forget that the state lives at the expense of everyone."

- Frederic Bastiat

No, I would expect them to harbor about the same sentiment. But there would be half as many gays; so, only half of the previous number of gays would be experiencing the oppression, prejudice, and psychological discomfort throughout their lives.
You are entirely correct. It would also take an extremely long time because it would take multiple decades for only prohibiting sperm donations to have effect on demographics in any significant way.
In other words, they're heterosexual but carry genes linked to gays; it's a recessive trait? Of course they would donate sperm like normal. But the procedure would still significantly prevent/reduce the number of the self-aware gay donors. I imagine that if people are aware that taking an action would be breaking a law, they wouldn't go out of the way to do it; they would be much more reluctant if they knew before they went to the donating place.

You check it for the HIV as anyone elses and then store it. What is the problem? Do you really think it is going to make gay babies? That would really be insane thinking.

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I have never said anything about HIV. I would imagine that any reasonably intelligent donation place would check for it, though.
It's not necessarily going to make gay babies, but it is more likely to produce gay babies, because of the very likely genetic link.

Even if it doesn't, it will still increase the frequency of genes in the gene pool that contribute to being gay, even if the carrier of the genes doesn't feel homosexual tendencies.