Materialism is literally gay. From the materialist perspective, "you are your body", and whatever mind or consciousness may be is created by it or a part of it (the brain, they usually say).
If a guy's body were to be changed completely into that of a girl like in genderbender hentai, or manga like Onimai, then from the materialist perspective it wouldn't be gay to be attracted to them because that's all that there is to being a girl, so it would be no different to being attracted to any other regular girl.
Let's now consider a guy who says that he's straight but that if he were to be genderbent into a loli like in the aforementioned works, he'd let all his lolicon friends fuck him through every hole, not because he'd like it, but as a generous favor. All sensible people can recognize that everyone involved in such a scenerio is gay.
Basically, materialism also functions as a metaphysical cope for faggots to defend their faggotry.