I haven’t read through the thread (but I will

) but I start with is that Church teaches that sex outside of marriage is wrong. Then I might add all the things that the Church ultimately teaches as disordered (contraception, pre-marital sex, extra-marital sex, pornography, masturbation, etc.) to emphasize that people who are gay are not singled out as “disordered” because that’s how they might understand it (this also knocks the homophobes of their high horse or at least gets their attention).
But for that very same reason, I think discussion of sex acts is beneficial to clarifying the matter. When you say that “extramarital sex is wrong”, and then you tell gay people they can’t get married to each other, they get awfully curious awfully fast about *why *they can’t get married. The ban on masturbation doesn’t explain
that, and appealing to the wrongness of extramarital sex is circular. So what’s wrong with gay marriage?
My answer would be that there is a problem with particular sex acts. And I have experienced that for myself: that certain sex acts are not only not lifegiving, but they also are self-centered. It’s not that the person involved with the act has to be selfish, but rather that the activity itself tends toward the selfishness of at least one of the partners.
The marital embrace – done well

– is not that way. The act itself becomes more and more wonderful the less you care about your own pleasure.
When you pin the problem on the sex acts, the philosophical basis for the opposition to gay marriage gets a lot clearer, I think. But then there is a problem: modern marriages are being destroyed by sexual selfishness, characterized by an obsession with oral and anal sex. And even the Church seems to be OK with this, to some point, since they are (at best) looking the other way while it is taught that these things are morally licit, with certain qualifications.
But nobody listens to the qualifications. They just think – as you suggest – that sex acts don’t matter. If you’re married and open to life, you’re good. But I just think that might be dangerously false.