Allowing gay “marriage” to be legal would be representing it as an equal goal for one to seek to attain if they have same-sex attractions. This would put their soul unnecessarily in danger, especially compared to good catechesis from a young age teaching that attractions should not be acted on but rather used in manners appropriate to moral law. Thereby, same-sex attractions should be channeled into close, non-sexual friendships with members of the same sex that do not tempt you into sin. Marriage (and parenting, going hand in hand) should also be taught as a spiritual and social responsibility, not as the endgoal of a temporal seeking of Earthly pleasure.
In short, whether or not Churches would be forced to do anything is irrelevant. Even if the Catholic adoption agencies could remain open, and even though churches in the US will never be forced to minister to gay “weddings,” legal same-sex “marriage” will always be opposed by the Church because of the intense risk it puts on the souls of those who have same-sex attractions, particularly those born in non-Catholic families or with Catholic parents who choose to not put effort (or who have a lack of knowledge themselves from which to do so) in their child’s formation.