There's a hack emulator for Harvest Moon that allows for gay relationships.
Stardew Valley also allows for NSFW gay content since it's moddable. >>5278
Generally if the game is MODDABLE, there will be room to incorporate gay content if there aren't any and to amplify it into NSFW territory, should you choose to do so.
Of course mainstream games like, Dragon Age Inquisition, the Fallout Series, the Mass Effect Series, Skyrim, Sims 3&4, all allow homosexual relationships; graphic sex, full nudity if you mod it.
Personally, unless the game has explicitly advertised itself as a "gay sex game", no casual indie games come to mind. You could probably compile ~50-60 Japanese Bara games that uses the RPG Maker/Visual Novel format and has explicit gay content. There's the odd furry games funded on Patreon like Blackgate and Extracurricular Activities but I'm not too heavy on that scene so you'll have to crawl through sites like U18 for more.
Coming Out On Top, Bastard Bonds, See No Evil are a few recent NSFW indie games that come to mind. Post too long. Click here to view the full text.