When it comes to the OT I think it is important to remember that people often pick and choose what they want to believe which to me is plain wrong. Either you accept (like me) that Jesus fulfilled the OT and it doesn't apply anymore or that the OT should still be followed to a tee in conjunction with the NT. That would mean you believe in being put to death for mixing two kinds of fabrics together as well as a bunch of other stuff because Jesus never explicitly said that really wasn't required anymore.
That brings us to Gay Marriage. I'm somewhat of a moderate on this. It is my belief based on scripture that marriage is reserved for a man and a woman for procreation. However it is also clear that the concept of, "marriage" (I'll use quotation marks from now on when not referring to the Christian concept) isn't really followed at all in the Western world. True marriage is a lifelong vow to God that can only be invalidated if one partner is unfaithful (I include abuse in that as well as). In my mind the only true marriage in God's eyes can be between a man and a woman who makes vows to God. I think heterosexuals such as John McCain, Donald Trump, Bill Clinton, Kim Kardashian and many others who divorced or committed adultery made marriage a mockery. Indeed, Western society seriously underestimates the importance of marriage.
With that said however, I'm far from believing Homosexuals and Bisexuals are going to straight to hell and we should help them on their way by stoning them. For the latter, it should be emphasized that marriage to the opposite sex is much more preferred and for the former monogamy is the only acceptable form of relationship (for what's it worth the same applies to heterosexual people. It's clear God wants us to have true husband/wife, not have as much sex as possible with as many people). If an LGBT couple truly love God then I am in favour of them declaring vows to each other in front of Priest to dedicate their relationship to God. So basically the equivalent to, "marriage" in modern day society but not Christianity. Oddly enough I am in favor of adoption but not IVF as I think the latter is a selfish act for anybody to commit and is not out of love but desire (that applies to everybody).
With all that said and done, we should always welcome with open arms our LGBT brothers and sisters if they truly believe and are willing to live a Christian lifestyle of monogamy and dedication (I'm not saying that they are inherently lustful but that that is the Lord's teaching on sexuality). Remember Paul said it is actually better for those who can to not have any sexual relationships as well to fully dedicate themselves to God.
God is love and we must strive against letting any hatred in our hearts.