What about polygamy, adultery, prostitution? If what consenting adults do behind closed doors is a fundamental right, then you could not discriminate or criminalize any of those activities.
Using your homosexual analysis, you could not pass laws against drug use.
How do you allow for the criminalization of polygamy or prostitution or the prohibition of homosexual or insestuous marriage under your analysis?
I don't see that it is possible. If you cannot constitutionally discriminate against people who practice one form of adult consensual sexual deviation because it violates the equal protection clause, then you cannot discriminate against any other group on the basis of their peculiar sexual deviation.
Leaving aside the question of whether or not this is true, this most certainly not is what the Colorado anti-gay statute did. That statute, rather, singled out one group performing a completely legal activity, and said "Hey, you have no anti-discrimination rights at all." Well, under the Constitution, that violates the Equal Protection clause.
Now, as regards polygamy, adultery, and prostitution, well polygamy is easiest. The State still defines marriage, and as long as marriage is not discriminatory (e.g. the old anti-miscegenation statutes), then marriage is okay. Adultery laws are impossible to enforce, would be bad public policy, and would require such an invasion of privacy that it would undoubtedly be unconstitutional to enforce. Prostitution, even when legalized, tends not to be an arms-length transaction between equals - a lot of the girls are coerced into it by their pimps.
If you cannot constitutionally discriminate against people who practice one form of adult consensual sexual deviation because it violates the equal protection clause, then you cannot discriminate against any other group on the basis of their peculiar sexual deviation.
You cannot constitutionally discriminate against people who practice any legal form of adult consensual sexual deviation.