Patriarchies are predicated on a gender binary where men and women are polar opposites and men and masculinity are envisioned as superior. There's also a small box of acceptable behaviors and norms in order to be "a real man." Homosexuality messes with that binary and feeds into Greco-Roman era ideas about being a "receiver" of male sexuality. They had no problem with same gender sex, but the guy who wasn't doing the penetrating was seen as essentially about as manly as a woman.

The same attitudes are still around today. Simple as that.