Me and my husband are very, very close emotionally, BUT ...?

... when I think about having with him in any way, shape, or form, it repulses me because it makes me think of, for instance, that I'd be having with my brother. He is very attractive, so I don't know why I feel this way. We are also very close and inseparable, like we're twins (I'm an only child), and we love doing things together like traveling and watching movies. He does not seem to mind this problem. I'm 36 and bi (which my husband affectionately refers to as "status quo"). I know I'm not impotent, and I know it's not him. We still kiss, hold hands, and cuddle, but the thought of with him feels so "wrong". The straight I have had before (with him and two other men before him) was mediocre at best. I know I'm not because I'm still attracted to men (although very sporadically).