From my understanding of how to tag, https://e621.net/posts/2600334 should probably be tagged male/ambiguous. I went go change it but, since the source denotes it as gay, I tried to add male/male_(lore) which doesn't exist. Should I leave it alone? Only one character is obviously male. Or am I misunderstanding what _(lore) tags are for?

You are right, there isn't a male/male_(lore).
It should probably just be tagged with male_(lore) + male/ambiguous.