<human>: Can you give me the gist of the text in a nutshell?

My sister and brother don't get along very well, because they're both stubborn and throughout our childhood each went out of their way to make the other miserable. Now that we're older, they get into political debates a lot. A recent argument of theirs confused me. 

They got to talking about LGBTQ issues, and it immediately turned out that my sister was arguing things like, "born this way" and my brother was arguing that it was a choice. There were also several more key talking points where my brother made it clear he did not support LGBTQ issues. It got to be a very heated argument, and I was kind of sad to see that my brother was taking that position, because I believe him to be a very intelligent and caring guy. 

However, a short time later, my brother came out to me as bi. He hasn't told anyone else in our family because they wouldn't understand and it's none of their business. He's happily in a poly relationship with an engaged couple. Cool. Good for him. 

However, that made my brother and sister's previous argument really confusing. Did he argue against her just to be a contrarian? Does he truly believe, like the rest of my family, that being gay is a choice you can choose or un-choose? Maybe he thinks heteros are disingenuous by saying they care about LGBTQ issues? Or is he maybe struggling with self acceptance? And if so, how can I best support my brother?
<bot>: My bi brother argued with my sister against LGBTQ rights and issues. What's going on with him? And how can I be supportive of him?