I don’t see why an honest, charitable discussion would not be permitted.
For #1, I think the natural law argument is quite strong. Sex serves a purpose, and sexual acts contrary to sex’s final end is disordered. The Church also teaches that sex is only appropriate within the context of marriage. Granted, the second argument reaches (I think) a bit beyond a basic understanding of natural law, but it is nonetheless a Church teaching. The former excludes homosexual acts, and the latter (combined with the former) exclude homosexual “marriages.” Now, it seems that the former is fundamental. We can discuss that aspect. But so long as the natural law argument regarding the former is intact, I don’t see how any discussion of gay “marriage” can proceed.
For #2, given the Church’s understanding of objective truth, I don’t think it can teach otherwise. Unless you think the Church is wrong with regard to #1. There can’t be both unitive sexual activity open to procreation within the context of marriage or arbitrary sexual activity of any sort regardless of marriage (the opposite extremes). The Church teaches all 3 must be present (unitive, procreative, and within marriage). If any are missing, it is contrary to Church teaching. Can it be otherwise? Only if you deny the unitive, procreative, and/or the marriage requirements.