things intrinsically unequal

don't have to be passed as equal.

   There is a lot of passing for and posing going on these days.
Jenner wants to pass as a real woman (damn his chromosomes), and a Seattle NAACP director wants to pass as black.
   Homosexuals want to pass as healthy married couples in "marriages."

Homosexuals are not the same as heteros in marriage because definitionally homosexual marriage must deny their internal biology to the extent that they cannot with just themselves create human life. Heteros who do not deny their chromosomal make up or internal reproductive biology can create humans just by themselves. If homosexuals want a family with children one or the other or both of them have to borrow someone else's. So definitionally a family will only be possibly constructed with other people- other people who may have a basket of rights with regard to their progeny. When a bisexual man leaves his wife and kids and fights for them in custody to live with a man, he takes someone else's children (or at least in part.) When someone rents a womb of a surrogate they take someone else's child-someone else grew it in their womb, they borrowed genetic material from at least one other person to make it- all those people might have rights.

   There is no requirement to treat unequal things equally. People who deny their internal biology do not have to be legally treated equal with people who do not. And no, this is not the same thing as old people who marry, because old people who marry are not denying their biology definitionally they are honoring it. They know they can't have kids so adopt or don't-just accept they are too old to procreate- but don't insist on using one of their genetic material to try to create life with someone elses. Gay people must deny their biology to have children (at least one of them must.) Why is this different? Because we want to give people who can within their own families just by themselves create life and raise their own kids preference to do that over people who choose to take someone else's or break up a family (in the case of a gay man for example leaving for a man.)

To do this imposes an artificial false synonym that ignores the other potential rights attendant to other people.

  Point 2-
To create a constitutional right of something culturally normalizes it. This happened in the abortion debate.
A tip of the hat to 'rare but necessary' abortions in 1972 has seen the normalization of the process, federal funding a massive corporation abortion mill and over 50 million abortions. People use it as birth control.
Gay marriage given a "just as good as" imprimatur will normalize it, not just tip a hat to the marginalized. It will provide an environment where all kids will experiment with gay sex "why not" because the culture says it is just as good as hetero relations.
You already see this in places where there is full cultural acceptance, In places like California where gay everything is in your face, there are statistically way more youth who consider themselves bisexual or gay than in an other part of the country save perhaps DC or parts of New York (where cultural acceptance is high.) Where there is high cultural acceptance and no normative weight placed on one over the other people experiment every which way and come out everything.

This attitude, you might recall (or research if you haven't heard) was surfacing during the decadent demise of the Roman Empire.

   Creating a legal fiction of equality when none exists in fact is problematic for a number of reasons, not least of which, we want to create social conditions whereby people honor their own internal biology as reflections of worship of the ONE who Created them.