I'd prefer in that situation for the two to be friends. It would be nice to be comfortable enough to be open to a threesome on a rare occasion if all parties thought it would be a positive (possibly moresomes I guess if I really liked one of their partners), but not involved romantically/sexually with each other - triads are just so difficult. I don't need to be stressed out with that dynamic, worrying if they are fighting, letting their relationship impact negatively on me, or having my relationship with either of them affected if they have problems or break up.
I wouldn't have an issue if they were bisexual, I just wouldn't want them to be involved _with_ each other while involved with me.
I watched my share of gay male porn in my time and it hasn't ever really done anything for me. Then again, watching any romantic partners even just making out makes me feel like I'm invading their privacy, so I get the feeling being involved in a triad would be an awkward thing for me in general, no matter what sex each partner was.
Happiness will never come to those who fail to appreciate what they already have.