Genesis 2:24For this reason a man will leave his father and mother and be united to his wife, and they will become one flesh.
Eve had no father to sell her to Adam.
Here you falsely represent (either intentionally or by ignorance) what the Bible says about marriage. It does not state or consecrate the method or circumstances that brought a man and a woman together. It only defines that relationship between a man and a women after they have been joined.
As for the "shellfish" argument as it is called, again you lack understanding of the Bible, the laws of Leviticus are divided in to those that are moral, social and ceremonial. The ceremonial laws defined how the Hebrews as a people were to worship. The social laws where meant to separate the Hebrews from the nations that they lived amongst. The ban on eating of shellfish and animals with cloven hoofs, that didn't chew their cud were social laws. These laws (ceremonial and social) were set aside when Christ opened the door for non Jews to become part of the chosen people.
The moral laws have never changed. Homosexuality is banned by the moral laws. If you take a closer look at Leviticus 18
You see the ban on homosexuality is snugly nestled between the ban on child sacrifice and the ban on bestiality. To apply your logic to scripture then you would have to agree that child sacrifice and bestiality are OK because Christians eat shellfish.21 “‘Do not give any of your children to be sacrificed to Molech, for you must not profane the name of your God. I am the Lord.
22 “‘Do not lie with a man as one lies with a woman; that is detestable.
23 “‘Do not have sexual relations with an animal and defile yourself with it. A woman must not present herself to an animal to have sexual relations with it; that is a perversion