You can divide experiences into four categories - gay-exclusive, straight-exclusive, possible for gay and straight, and possible for neither. Do you deny that the gay-exclusive category is larger than the straight-exclusive category? You you deny equivalent statements for race/gender?
Yes.
Well, here's what I can think of for the straight-exclusive category:
-Being attracted to someone of a different gender
And here's a partial list for the gay-exclusive category:
-Being attracted to someone of a different gender
-Having limited access to marriage and legal recognition of relationships, depending on country
-Having limited ability to adopt, depending on country
-Condemnation of sexuality from religious organisations
-More common allegations of pedophilia and child abuse
-Multiple organisations and government support dedicated to promoting acceptance of one's sexuality
-One's sexuality commonly being used as a derogatory term
-Restrictions on occupation (such a military service), depending on country
-Study of one's sexuality considered an academic field
Anything you'd like to add to those lists?
You seem to view homosexual experiences as somehow more exclusionary to those who aren't gay as more exclusionary than hetrosexual experiences might be to someone who is, and I just don't see any rational reason for it.
Suppose you're straight, and you make a list of all the experiences you've had that are impossible for a gay person to have. Now suppose you're gay and you make a list of all the experiences that are impossible for a straight person to have. I'd expect the gay person's list to be longer.
If I'm reading a book with a gay protagonist then I might be able to sympathise with him, but any mentions of romance or sexuality are likely to alienate me from the character. It doesn't line up with my preferences so is going to push me further from the character and be less likely to enjoy the scene. Generally it's like reading a book with the protagonist holding any sort of views I disagree with, which can be enjoyable but makes the sections where those are important harder to get through and less comfortable as a method of escapism.
Why would that be any different for a gay person reading a book about a straight protagonist?
A story about a straight person and their romantic interest would include some straight-exclusive experiences (mostly the direct physical attraction), and lots of experiences common to both straight and gay people. A story about a gay person and their romantic interest would have the same common experiences, but would include at least as many gay-exclusive experiences.
Further to all that there is the simple matter of under-representation. If I'm reading a book about a gay character and have a little trouble identifying with him, it's not a big deal. I have a dozen straight characters who I can turn to for more comfortable romantic escapism. Hell, odds are good there will be one in the same book. The gay reader is far less likely to have somewhere else to turn.
It's about being able to identify with the character and want to think myself into their position. Same-sex romances (done in a substantial manner) make that harder for me because that's just something I don't relate to. I might be able to appreciate the character, sometimes more than if they were just another cookie cutter dudebro, but I'm less likely to think of them as a proxy for myself. That's what I meant by alienation.
But there are far more elements of a romance story than the physical attraction. One can identify with the vast range of sexuality-common elements even if the element of physical sexuality is not shared with the reader.