TK421:
There may or may not be a genetic/evolutionary purpose for it, but that purpose isn’t creating facsimile marriages that parallel the marriage between a man and a woman.
That is not the question. The question is whether homosexuality is immoral or not if there is evolutionary cause.
But to respond to your question, explain why evolution justifies marriage then between a man and a woman, but not two women, or two men? Your comment is a red herring. Sure, evolution may not justify homosexual marriage, but it doesn’t justify heterosexual marriage then either.
Your underlying assumption appears to be that if a behaviour or mode of being comes about as the result of evolution it is de facto morally permissible.
That seems to be your assumption despite that you phrase it as a question.
The question is whether homosexuality is immoral or not if there is evolutionary cause.
Unfortunately, that would be an instance of what is called the is-ought problem or naturalistic fallacy. Merely because something
is does not, thereby, establish that it
ought to be morally accepted – even if some arbitrary threshold of 10% is achieved.
The ought question is one of morality not biology, evolution, or even survival.
So, to answer your implicit question: an evolutionary cause does not mean anything regarding the morality or immorality of homosexuality. That has to be argued on moral grounds, not genetic outcomes.
Genetics/evolution might (although it doesn’t currently)
explain homosexuality (the condition of being attracted to the same sex) but
justifying homosexual behaviour is a different question, as is whether same sex marriage is morally justified. The answers to both of those come as the result of moral deliberation, not evolutionary change.
If some gene mutated such that 10% of human beings became murderous to everyone around them, we wouldn’t claim murder is thereby justified as a moral behaviour. We might excuse those afflicted with that gene from being morally culpable, but that isn’t the same as accepting murder as a moral act going forward.