I guess it becomes a question considering the act of consummating a gay marriage, sodomy. A serious sin, as noted in the OT & NT Bible. 

So, what's wrong with a priest preaching against it?

As to weather it's the #1 sin in US may be up for grabs, but I would think many hearing the a priest speak against adultery & fornication, would label the priest in the questionable language you post here.