I will assume, without more to go on, that you are liberal or to the left of moderate. My apologies for labeling if I am wrong. I'm writing to correct misconceptions that I perceive you may have. If I make points here that you already know, I'm sorry about that; but I need to clarify what many in the media and on the political left (and even moderates) do not know or understand. In fact, many devout Christians do not get it because many do not fully understand their faith.
FIRST, but not the main point, the harsh stands in the Old Testament cannot be held against Christianity because that old book was to an extent for the benefit of ancient peoples who needed harsh rules from their God and religious leaders, as they would engage in things like incest and such (like many barbaric ancient peoples). The Koran can be a greater indictment of Islam (because it is current doctrine) than the Old Testament can for Christians.
The redemption of all sinners by the sacrifice made by Christ replaced the harsh punitive practices of the Old Testament. But the book is still valid for all it taught. The future coming of Jesus is predicted in that older book many times in many places in the text. It is not invalidly archaic.
SECONDLY, and foremost to this discussion is that the core doctrinal teachings of Christianity are not debatable or negotiable. You or the government can try to force them to be, but the effort will be meaningless. Many Protestant denominations will perhaps cave under great pressure, but that will not change scripture, the Law of God, or the beliefs and actions of true Christians.
On a side note, we Catholics (whose Church was founded personally by Christ, unlike Protestant institutions) do not really consider the Protestant churches to be fully valid, though we accept their Christian membership as Christians. This is why we capitalize "Church" when we reference our own and not when we reference a Protestant denomination. Christ founded His Church for there to only be one, universal Church, not many ['catholic' (small 'c') means universal].
THIRDLY, because the core doctrine cannot be negotiated, it cannot be changed. We believe that the core doctrine comes directly from God. He could change his mind if he were not perfect. But, being a pure being, he got everything correct from the beginning of time. Thus, there is nothing to change. We cannot change what we did not create, God's Christian doctrine given to us. We will not change it nor obey laws that demand that we do. The civil unrest I see on the horizon, should such demands be made, is mind boggling and scary for me to contemplate.
Homosexual people have their secular rights, but should not make demands on Christians, Jews, Muslims, or any religious believer who does not accept their romantic behavior. There are plenty of businesses that will serve them. Let them search. As to marriage within the Church, no true priest or Protestant minister would perform one; and, if they did, such marriages would be invalid in the eyes of God. Homosexuals, we Christians do not want to assist you in your sinful behavior, nor are we required to. Certainly there is no moral requirement within our Christian faith to do so. No, it is exactly the opposite: we are bound by teaching and belief to stand against homosexual behavior. But unlike you homosexual advocates and meddlesome liberals, we respectfully speak out in a nonaggressive way. This is a core belief liberals are trying to mess with, and the 'Christians' who yield are revealing their weak faith. It would be a sin for us to do so. Not only that, but many images, designs, messages, and musical presentations your wedding, party, or whatever may require might be sinful and repulsive to us and for us to have to help create. This is not comparable to racial and ethnic discrimination, because this involves your sinful behavior, not your color or appearance.
We are often asked to prove the stand against homosexuality in scripture. Here it is (the second one in Christ's own words):
[Old Testament _ Genesis 2:24]
Therefore a man shall leave his father and his mother and cling to his wife, and they become one flesh.
[New Testament _ Mark 10: 5-9
But Jesus said to them, "For your hardness of heart he wrote this commandment. But from the beginning of creation, 'God made them male and female.' For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh. So they are no longer two but one flesh. What therefore God has joined together, let no man put asunder."
It is notable that the Old Testament reveals a rejection of homosexuality in an everyday practical way, reinforced by repetitive application, practical and tangible elements of life that secular liberals can identify with. Historically in that old volume, each time God's judgement is visited upon his chosen people, it is because of their ungodly behavior and lack of proper acknowledgement of him as their God. In these times, there are generally two breaking points or tipping points that bring about such judgement: the worship by the people of other, false gods and homosexual behavior (and homosexual prostitution). The historical scenarios vary, but generally this is consistent.
Marriage is one of the seven Catholic sacraments, the most holy elements of our faith that we can personally be active within. As the scriptures say in both parts of the Holy Bible, it is between man and woman and blends them in a spiritual way together as one. This is God's action and he cannot be superseded. Male to male and female to female are rejected clearly by their omission.
These words of Christ also disallow divorce. Christ's words above were spoken when he as asked about divorce and the fact that Moses had allowed it. That leader of the Jews had allowed it, "For your hardness of heart..." (their poor practice of marriage and keeping of their vows). Catholic annulment of marriages is based upon the fact that certain marriages are deemed to not have been godly and, therefore, the sacrament was not valid.