Pretty much asked the question in my title … having several discussions on this in various places, and was wondering if the Pope, by himself, can change a law of the Church, and suddenly allow gay marriage?
The Pope has no power to change the Church’s teaching on same-sex “marriages.” Homosexual acts violate the natural moral law and thus are intrinsically immoral. As such, the Church could never change its teaching that such activity is wrong. The Catechism of the Catholic Church states in paragraph 2357:
Basing itself on Sacred Scripture, which presents homosexual acts as acts of grave depravity,tradition has always declared that “homosexual acts are intrinsically disordered.” They are contrary to the natural law. They close the sexual act to the gift of life. They do not proceed from a genuine affective and sexual complementarity. Under no circumstances can they be approved.
If homosexual behavior contravenes the natural moral law, then it logically follows any sort of union that embodies such behavior contravenes the natural moral law as well.