I'm trying to think of what the tag name would be for porn where a straight male is unwillingly raped by a gay male.

Not certain there needs to be a tag.
orientation_play male/male rape

There's straight_to_gay, at least.

Might want to consider supplementing the tags Lafcadio mentioned with rape oriented tags (such as below). But this sounds niche enough that it's something you'd make a set for.

explicitly_stated_nonconsent,questionable_consent, until_they_like_it, imminent_rape