We shouldn't judge them, unless they are inside the church.
(I Cor 5:9 I wrote to you in my letter not to associate with sexually immoral people— 10 not at all meaning the sexually immoral of this world, or the greedy and swindlers, or idolaters, since then you would need to go out of the world. 11 But now I am writing to you not to associate with anyone who bears the name of brother if he is guilty of sexual immorality or greed, or is an idolater, reviler, drunkard, or swindler—not even to eat with such a one. 12 For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? 13 God judges those outside. “Purge the evil person from among you.”)
That is not to be taken as endorsing thier behavior either. We should not condon gay marrage.
What we should be doing is clearly preach God's redemption through His Son Jesus Christ to all.
A great book on the subject is Secret Thoughts of an Unlikely Convert. An English Professor's Journey into Christian Faith
This is a deeply moving book that follows a radical feminist lesbian's journey to the Reformed corner of Christianity, ultimately to marry a Reformed pastor. In this book we can learn much how to deal with all sinners and come to a deeper understanding that it is God who saves.
AMEN!