Seed for nothing

Miki,
Do you think there is a halakhic prohibition on this matter?
If so, what is it?
What is its origin?
What is its validity?

Why not? The things are ancient and you can find them with a simple search on the Internet.

Does the prohibition not stem only from Kabbalistic sources?
As the Hatem Sofer states? “All Western Kabbalah in Halacha is obligated because it sows hybrids”…
In short, maybe this is a prohibition of Legumes 2? (Unsubstantiated)

It's not just Kabbalistic. See Nida 13:2. I directed you to search on Google.

Since the prohibition of semen being wasted is that it is spilled in a place other than its proper place, namely the uterus, and since homosexuals are exempt from the commandments of fertility and reproduction (I have read this in several places and if I am not mistaken, your opinion is the same), it occurred to me to ask this question.

Where did you get the idea that homosexuals are exempt from this commandment? And is there some sermon that I didn't know about?

From the verses in Genesis it seems that the whole problem of a sperm being void is this: because he did not want to establish a sperm for his brother. However, for a single man who does not establish a sperm, is there a problem?
And yet, except for the time of ovulation, every time it will be forbidden. And what do you say about a woman who is in the womb, which is permitted?

There is room for such an interpretation, but this is not the opinion of the poskim. Using the vagina during menstruation is no different from using it on days without ovulation. When there is a vagina, it is permissible to use it even when it does not lead to fertilization and reproduction.