Never (at least not on the institutional level; I'm sure some priest somewhere has done it already though). The Church's teachings are not subject to popular vote or judicial decisions; nor is the United States particularly important. If those in the Church hierarchy are Catholic (ie. that they believe in the Catholic Church's teachings), they cannot then turn around and change said teachings. It is true that the circumstances of society when an act is committed may have some bearing on whether or not it is wrong; ie. charging interest in Jesus's time was indeed usury but it was usury because of the way it was carried out not because charging interest is intrinsically evil itself. Given the purpose of sex as a marital act and the entire idea of a sexual relationship pointing toward that end, the Church's teachings on homosexual sex acts and gay marriage are settled. Matters of prudence, disciplines of the Church, and other rites as such can change to meet the needs of modern society but the truth cannot for Jesus Christ is the same yesterday, today and forever (Heb 13:8
).
On this point, here's the translated text of Pope Francis's famous "Who am I to judge" response on the plane:
His response, interestingly, is that the existence a lobby, for any purpose, is the problem. The problem is less homosexuality, or any other particular change people want to make to the Church's teachings, but that they want to make a change at all.