Well, because they love each other and have a happy family. It makes sense to me that they can be in love and intimate, but many other people don't understand. They're both very open about the nature of their relationship, and though they recognize it as unusual, they don't have any shame about it. He is very supportive of her, and they're both active in the GLBT community. Lesbians I encountered weren't interested in being with a bisexual, because they always assumed I'd leave them for a man, as if that were my default preference.
Bisexuality brings the problems of being gay, and then some.
Back when I had a sense of humor about it, I'd joke that telling men I'm bisexual is the best way to pick them up. But I see it in darker terms now, imagining how others could view my decision to be with a man. If I choose a woman, I must be gay and the bisexuality ruse was just because I couldn't admit to being gay. It's a special kind of prejudice, the bias against bisexuals. It's as if bisexuality isn't legitimate to many people, straight and gay.