It’s a debating technique the left uses, implying if you believe one thing the Bible says you have to believe EVERYTHING it says. Google “Letter to Dr. Laura” for examples.
The flaw in the argument is more fundamental. Rules and customs about goats and such are not the definition of marriage on any view. The argument is not remotely reason based, its a poor rationalization of the bigotry of the ones making the argument made up to appear like a reasoned argument.