The contradiction between opposition to gay marriage and Christians not adhering to the Mosaic Law fails on more than one level. We seem to have this thread every few months.
First some backround…
The Law of Moses, Mosaic Law, or Law Code (interchangeable terms) were a combination of laws that ultimately numbered around 600 laws. They touched every part of Israelite life and governed dietary, sanitation, dispute resolution, ethics, relations between man and women, sexual matters, trade & commerce, sacrifice, worship and many other things. It’s worth remembering that the Nation of Israel was a theocracy. It’s leaders were religious men who were said to have received these Laws from God Himself. (Starting with Moses, of course)
For some reason, many people remember the prohibitions about shellfish or mixed thread clothing, and don’t realize that those laws actually numbered around 600—a very comprehensive and pervasive set of laws that touched every part of Jewish life. Every part. Many people don’t realize that The Ten Commandments were part of the** Law Code.**
It is not as if modern day Christians have repudiated the prohibition on shellfish (and the other laws), but that the early Church—starting immediately at the death of Christ—made this change and ended many of the requirements of the Law Code. The proponent of this sea change? Paul himself.
Whether he was right or not, whether he had the authority or not, Paul’s reasoning is clear and he made his case clear in his writings to the congregations.
According to Paul, the whole purpose of the Mosaic Law was to “lead to the Christ”; to essentially prepare the Jews for the Messiah. You can find Paul’s reasonings about the purpose of The Law, and his views on how the Jews should view The Law* (after the death of Christ) *in the following scrpitures: (It must be remembered that he was not talking to “Christians”; he was talking to Jews. All of the Jews were in expectation of The Messiah. Both the Jews who rejected Jesus as a nutcase, and those who saw him as the promised Messiah thought they were following correct Judaism)
His view as to the purpose of the Law:
Galations 3:24 24So the law was put in charge to lead us to Christ[a] that we might be justified by faith. (and others)
His view as to how Jews should now view the Law now that Christ had come:
Galations 3:25 25Now that faith has come, we are no longer under the supervision of the law.
Romans 10:4 *4Christ is the end of the law so that there may be righteousness for everyone who believes. *
Romans 7:1-6 1Do you not know, brothers—for I am speaking to men who know the law—that the law has authority over a man only as long as he lives? 2For example, by law a married woman is bound to her husband as long as he is alive, but if her husband dies, she is released from the law of marriage. 3So then, if she marries another man while her husband is still alive, she is called an adulteress. But if her husband dies, she is released from that law and is not an adulteress, even though she marries another man.
4So, my brothers, you also died to the law through the body of Christ, that you might belong to another, to him who was raised from the dead, in order that we might bear fruit to God. 5For when we were controlled by the sinful nature,[a] the sinful passions aroused by the law were at work in our bodies, so that we bore fruit for death. 6But now, by dying to what once bound us, we have been released from the law so that we serve in the new way of the Spirit, and not in the old way of the written code.
Ephesians 2:11-16 11Therefore, remember that formerly you who are Gentiles by birth and called “uncircumcised” by those who call themselves “the circumcision” (that done in the body by the hands of men)— 12remember that at that time you were separate from Christ, excluded from citizenship in Israel and foreigners to the covenants of the promise, without hope and without God in the world. 13But now in Christ Jesus you who once were far away have been brought near through the blood of Christ.
14For he himself is our peace, who has made the two one and has destroyed the barrier, the dividing wall of hostility, 15by abolishing in his flesh the law with its commandments and regulations. His purpose was to create in himself one new man out of the two, thus making peace, 16and in this one body to reconcile both of them to God through the cross, by which he put to death their hostility.
Colossians 2:13,14 13When you were dead in your sins and in the uncircumcision of your sinful nature,[a] God made you b] alive with Christ. He forgave us all our sins, 14having canceled the written code, with its regulations, that was against us and that stood opposed to us; he took it away, nailing it to the cross. (and others)
All texts NIV; highlighting mine.
These changes were a big change, and took some getting used to. In fact, the dispute in Acts about circumcision had everything to do with this issue—some of the “Old Guard” were struggling with the idea that new converts weren’t being circumcised according to the Law of Moses. Paul interceded and reiteretd that the Law Code was no longer binding. (which included shellfish etc etc)
However, I can’t see how any of this has to do with same sex marriage. My only contribution to the OP is to point out that the relaxation towards shellfish etc has been around for a couple thousand years. And in spite of the personal reasonings these threads inspire, the bible does have a lot to say about the subject. But it is clear that the abrogation of Law did in fact take place, and that Paul laid out his reasonings clearly and more than once. There really is little reason to speculate—the bible answers the question without the need to guess.
