It's a sad fact.
If anyone were to ask these specific Lutherans, or Presbyterians, or Anglicans, their response would be that they aren't going against the Bible one bit. Their position is that the passages in regards to Sodom and Gomorrah was explicitly about abuse and the lack of hospitality. They'll explain that the passages about a man laying with another man was more Mosaic than divine because breeding was such a necessary function in those days. They'll claim that because Jesus never said anything directly about it, then that follows reason as to why the two former positions are right.
That is their reasoning anyway. So they will protest to the grave that they are being consistent with the Bible and that Christians who oppose it are either bigoted and/or misreading the Scriptures. My younger sister, who I love, unfortunately holds to this position as well. She claims she re-read the Scriptures very heavily to get to the bottom of this divide with what marriage is and says she found no legitimate passage that condemns it when looking deeper into the meaning of the passages.
...But I have to disagree with my dear sister. When Jesus did speak about marriage, there is a precise reason why it was always in regards to a man and a woman. The home, under holy matrimony, is meant to be the domestic church. It is where the husband's role serves as prophet (a representation of God's love), priest (someone who represents his wife and kids to God), and king (someone who holds responsibility for the welfare of his family).
What marriage is of itself can only require a man and a woman, devoid of energy outside of the marriage where it doesn't belong, devoid of divorce, devoid of selfishness, and devoid of foolishness.
People who think that it's based on homophobia are going for the easy response to shut the other side up. Homophobia is wrong by definition. However, rejection of homosexuality is right. The reason why we should be against same-sex marriage isn't because we simply find it gross or we don't understand...it's much easier than that...it's really just because we don't have the authority change what marriage is. It really is an institution created by God, and it really does remain static.
Whenever someone asks me why I oppose it, my answer is very simple: "It has nothing to do with the individuals. I just have no authority to say it can change. No one does."