According to the "sneaky bisexual" theory, bisexuality is beneficial because of you can seduce the fit dude maybe he'll let you sleep with his mate, and if he's fit he probably has a high quality mate.

One hypothesis is that gayness is from an evolutionary point of view "too much bisexuality". Similar to how one sickle cell gene is beneficial, but two is deleterious. According to this theory, evolution favors bisexuals, and sometimes produces gays as a side effect of that.