Thank you for this reflection. It would have helped me if you had cited the specific arguments to which you were responding; in lieu of such examples, this seems like a straw man. Here are the parts of your argument that I think could use some more thought:

“‘Gentile’ is a self-definition based on a permanent and unalterable reality that is completely hereditary.”
Except that Gentiles could choose to give up their “comletely hereditary” identity and could become Jewish. This involved an intentional repudiation of their cultural background (sound familiar?). Sometimes we distinguish between “converts” and “proselytes.” Which are gay Christians to be? What does Paul say about proselytes?

“Being a Gentile was only incidentally linked to practices that the church found morally problematic (i.e. idol worship, sexual immorality).”
Is this true? What are you saying a Gentile was? Is it a race? An ontological status? Is it a lack of relationship to God? A legal designation? My understanding is that Gentiles were non-Israelites. Jesus’s life, death, and resurrection re-oriented the traditional way of understanding their relationship to Israel.

“The Church’s inclusion of Gentiles in the 1st century involved welcoming persons. The ordination of self-affirming practicing homosexuals would involve condoning behaviors…”
The inclusion of Gentiles also involved condoning behaviors. No doubt sinful sexual behavior was considered worse than dietary behavior, but both were treated with scrupulous caution. Inclusion of Gentiles clearly changed moral-religious practice.

“The Gentiles were welcomed into the Church in the first century, but Gentile idolatry and sexual immorality were not.”
There should be standards for sexual morality. As you say, the decision to enter the church for Gentiles involved a change concerning their fundamental identity and allegiance. Specific practices had to change (worship of idols, etc.). Others did not. Certainly there is no record of anything like a “practicing homosexual” Gentile in the Bible (in fact, this term and this question are farily new to our age), and the question is how one’s sexual orientation is similar to the questions you raise.

You say that his analogy doesn’t “hold water,” but the whole point of analogies is that they are comparisons; by their very nature they will be inexact. It would be nice if Paul wrote a letter “To the Practicing Homosexuals,” but sadly he did not. In lieu of such a work, we must try to draw analogies between his situation and ours. The question of Gentile behavior is one such analogy. It is fairly clear that the Gentile conversion was a mind-bending, church-breaking (or maybe creating?) event that changed the whole way Christians understood Israel. In recent history we have seen other prohibitions similarly changed (slavery, women’s ordination, and many other imperfect analogies). How is the inclusion of gay members in the church more or less like these many other analogies? How do we do justice to these analogies?