For the exact same reason you would "obsess" over a Star of David on the door of a mosque. The thing that it represents is in complete opposition to the message of the institution. That's not to say that I think it's impossible to be Christian and gay at the same time, but it is definitely impossible to be Christian and an adherent to modern LGBTQIA+ ideology, which is what that flag represents these days. They are literally, diametrically opposed on a large number of key issues, and the only way to attempt to reconcile them is to just take the small number of compatible pieces of one of them and glue them onto the other one. It's not even about dislike or distaste either, it's about shit like the Bible explicitly commanding against gay sex or the Bible explicitly saying "your body is a temple" within the context of not altering/desecrating yourself, both of which are things that are explicitly endorsed by LGBTQIA+.