Answer by Jim Ashby:
A pope CAN'T declare such a thing because it's a docrirnal matter. Papal infallibility prevents it. There are multiple (and infallible) papal bulls against homosexuality. Changing course on homosexuality means contradicting existing papal bulls — which contradicts the doctrine of papal infallibility — which would undermine the authority, and thus, the legitimacy of the church.
Such a move would be unthinkable. The church is stuck with homophobia.