I don't know why she couldn't have done a photo with only her draped in the flag. Her husband could be in the photo, too, with his arm wrapped around her. This would both assert her bisexuality for that sweet social media attention and show that he loves and supports her.

Edit: Not to say that bisexual people can't talk about being bisexual if they're in a straight relationship. It just feels weird and attention grabbing to post something like this to social media.