Apples and oranges, and you know it.
No, more like tangerines and clementines.
Gay "marriage" and straight "marriage" with a second "spouse" while your valid first spouse (or partner #2's valid first spouse) is still living, are both cases of attempting marriage with someone you are not, per the Holy Scriptures, eligible to marry.
Why? Jesus Christ defined marriage as the union between one woman and one man for a lifetime. If you divorce and remarry then you are committing adultery. A union between two people, one or both of whom having been married before and the former spouse is still living, is not marriage as defined in the Bible. Neither is a union between two people of the same sex. Different types of apples, if you want to use the fruit comparison, but apples nevertheless.