Sorry, I've got to disagree
I've got to disagree with Suz on this. I happen to be rather devoted to nativity scenes myself, and I collect Italian nativity figures. The Fontanini collection of nativity figures goes on and on including not just Mary & Joseph and the Baby, but angels, sheep, goats, ducks, Roman Soldiers, Rabbi's, students, shepherds, goose girls etc. etc.
Part of the tradition and symbolism of the nativity scene is the expansive scene which includes all of God's creatures from high to low. The tradition of the nativity scene is, after all, rooted in St. Francis who was noted for his love for all creatures and for all humans rich and poor. The inclusion of gay couples in the nativity scene is therefore entirely appropriate unless you believe that gay persons are somehow excluded from God's creation---unfortunately some Christians believe exactly that---and if those Christians are offended by our presence in creation, that is just too bad. I don't think God agrees with them, and I think God would like to teach them otherwise.