As I’ve said before, I don’t judge them. What other people do in their beds is between them and God.
I am comfortable calling sex outside of traditional male/female marriage a sin because of how I, and centuries of church tradition, interpret the relevant scripture passages. How I live up to that conviction is between me and God, and how you live up to it is between you and God. After all, I could be wrong.
God doesn’t command me to judge people; he commands me to love them. (Jeez, I feel like I’m channeling Poly when I say that.)