Okey-dokey, let’s take a look at this.
Here is the Snopes article on Angela Bowie finding David in bed with Mick, naked; it’s kind of inconclusive.
If someone is bisexual, it means they’re attracted to both sexes. If that person gets into a committed relationship with another person, then they don’t necessarily stop being bisexual or stop being interested in people of the same gender; they are just faithful to the person to whom they’ve made a commitment.
Bisexuals can get married, have children, and still be bisexual. Bisexuals can be in committed relationships with people of their same sex, and still be bisexual.
Sexual orientation is best defined as a wide spectrum between strictly heterosexual and strictly homosexual, with bisexual somewhere in between. My guess is that Bowie falls somewhere between heterosexual and bisexual. Which is as good a place as any on the spectrum.