I get where you're coming from but I think it's quite realistic to not have a sexual interest in the same gender, but then meet someone that you are sexually interested in regardless of their gender. I've been on the receiving end of that before and I don't think it can happen just with looks, but if you get to know that person during a certain psychologically vulnerable or adaptable time in your life it does happen.
It's possible, but becomes extremely suspicious when it becomes so common that gay characters in BL seem to be, ironically, a rarity.
Similarly, if a woman in fiction does the cooking and cleaning, fine. But when it becomes a universal trend and women become rarely represented in other roles, it becomes a suspicious stereotype.
Simply put, I'm not questioning its possibility, but its suspect regularity within BL.
Great, the BL cliché "I'm straight but only have eyes for you xox" strikes again...