This is a question, to which we shall never know the answer. I am of the opinion that David Bowie was in fact, bisexual. It was demonstrated through his past relationships and the claims that have swirled around his figure for as long as I can remember. In fact, he most probably regretted the claims regarding his sexuality due to the heightened level of scrutiny that such matters tend to attract.
I was coming of age when David Bowie was at his peak in popularity. As shocking as it might have been back then, it was known and accepted among people my age that he was bisexual. I have never heard that he stated regretting any sexuality claims. If he did, perhaps he just regrets letting the world know about his sexuality especially after marrying a woman and becoming a family man.
David Bowie has accepted that he had a voracious sex appetite and in order to satisfy that he slept with as many partners of either gender. Just because he slept with partners from each gender, it does not mean that he was bisexual. He has had more stable female partners than male in totality which clearly states his preference as far as bedroom is concerned.
David Bowie was a master at using the media to further his musical career. If it meant bringing some perceived sexual classification to the table, I would not put it past him. Besides being an amazing musical innovator, his business acumen was severely underrated. The man will be sorely missed.