I think it is pretty clear from your post that you should leave the category as 'bisexual'. If nothing else but to bring more light to the fact that there are, in fact, bisexual people in opposite sex relationships. Bisexual people in opposite sex relationships need this book, they need the affirmation that yes, they are still bi, even though they are in a passing het relationship. By labeling your book this way, you are affirming that. That you are not defined by only your present relationship.