You miss the point at least two times:
First: While it’s true that you sexual orientation is probably not only genetically determined, but also has a lot of factors in early childhood, it doesn’t matter. You should be able to make any choices that doesn’t negatively affect other people (being offended is not really negative). Being natural doesn’t make something better as I would advice against eating all natural arsenic
Second: You are right that in a perfect world we would not need any movement to legalize sexual orientation. Sadly we live in an imperfect world we’re these stupid thing called „abrahamic religions“ exists. After they drummed up homophobia you need a movement to make it ok to love someone who doesn’t fall in the narrow roles that society wants to see. (As was for couples of different phenotypes)