I've never read that his sexual attraction to men was known by anyone. All we know is he liked to hang out at gay bars to select victims. We gay men are used to that. It's a common thing. We don't call the men who attack us, sometimes after starting to have sex with us, gay. I don't think anybody would find that to a reasonable position.

We know for sure Dahmer didn't identify as gay, quite the contrary, actually. He often talked about how he hated gay people.

As far as anybody knows, he didn't want to have sexual/romantic relations with men. Rather, he wanted to kill, and sometimes cook and eat, boys and young men.

I don't see how it's possible to call him gay given all that.

We don't call call Catholic priests who sexually abused boys gay. Because that's not what gay means. What Dahmer did isn't what gay means either.