Do you think you should be able to call the cops and have them charge someone with a crime every time he tells a lie? thoughtlessly says something hurtful? fails to adequately support the poor, the needy, and the Church? All these things and many more are immoral. Certainly the civil law must be Based on the natural moral low, but we only make civil crimes of those immoral acts which cause grave damage to society.
And yes I agree sodomy, contraception and adultery should be illegal.
I agree, the OP’s premise is backwards. It is not a matter of “gay marriage is illegal”.
In the 19th century civil governments began to make and keep government registers of the names of people who are married. (Before that the regulation of marriage entirely a Church matter.) The reason is that governments recognise that marriage, and especially strong and stable marriages, benefits civil society in many ways, by producing the essential new citizens and giving them the best chance of being emotionally, mentally and physically healthy and productive members of society. Hence it has laws which support marriage and give married people certain advantages, for the good of siociety as a whole including unmarried people.
The proposal is that civil governments should register sodomite relationships between people of the same sex in the same way as it registers marriages, and perhaps even CALL them “marriages”, and give unmarried people the same benefits that married people get, just because they regularly sodomise each other. (If anyone claims that this is not the reason, ask him if he would agree to the sdame “marriages”/partnerships being registered between two people who are not sodomising, eg two siblings sharing a household.) But it would be counterproductive for governments to do this, because sodomy does not give any benefit to society, but rather causes many ills and costss to society, so governments should discourage it, not encourage and support it.
Marriage has existed since the dawn of human existence. It long precedes any state, and even any religion, and no state has the authority to reinvent or redefine marriage.