Nope, that's not the argument here. The argument is whether the Texas sodomy law is "unconstitutional" because it allows punishes certain people for one kind of behavior, while letting other people engaging in the exact same kind of behavior get away scot-free.
If homosexuality is not a sexual gender, then where does the equal protection come from? Is it a race?
It is neither a "gender" nor a "race". It is a behavior that is usually ascribed to persons who are of the mindset to practice it. Not unlike "receiving Holy Communion" is a behavior ascribed to persons who are of the mindset to practice it, namely Roman Catholics.
If you are looking for a close equivalent to a class that is already protected by "equal protection", it would come closest to religion. They are both mindsets/sets of beliefs with specific practices related to those beliefs. To boot, both are "changeable" and are not fixed beliefs/behaviors.