Most of the gay H-games you'll find on here will have art inspired by 1 of 2 subgenres within Japanese homoerotic art—
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and
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. Essentially, it's a divide between masculine and feminine ("hunk" and "twink" in gay slang). Both of these have had a lot of influence on drawn gay porn, and are really well-developed, with yaoi in particular being developed because of its popularity among straight women (
fujoshis).
I think it also has to simply do with the fact that, with there being fewer gay than straight people, gay porn popular and well-liked enough to get put on here has to stand out. Anyone can make a quick buck selling mediocre straight porn to horny cishet guys, but if you want to make a decent living off gay porn, you need a bigger market share.