There are many other threads on this. Just remember that it promotes a general mentality of “if it’s legal there must be nothing wrong with it”, leading to dangerous public policy decisions for children. A couple of my more serious concerns are:
(1) It opens the door to adoption of children by homosexuals. Do we really want to hand innocent children over to people with mental disorders? This opens the door to sexual abuse, etc.
(2) It opens the door to intimidation, accusations of bigotry, and other forms of religious persecution against churches that will refuse to go along (i.e., perform weddings, etc).
ewtn.com/library/CURIA/CDFHOMOP.HTM
frc.org/get.cfm?i=WA03I35#WA03I35