It's important that they can do all this: ensures they're quite aware of who's a threat to their mate prospects. They might be bi, but just because they do this doesn't positvely mean they are -- perfectly straight women do this, too.
Conversely, guys don't seem to be as good at detecting each other's physical attractiveness, but can detect dominance (& a few other factors), seemingly because phys. attractiveness isn't nearly as much a threat as these other things.
Sex is a bigger deal to women as they've got more to lose --at least the potential of 9 months of resources and vulnerability. Guys can just move on 5 mins later. So, women need to spend more time selecting, protecting, and retaining a mate. Women seem more verbally open about sex than guys are -- it helps them get the information they need... I don't know that being more open about it, and 'checking out' other women makes them bisexual.
...or so the high-level evolutionary theory goes. Wanna read my upcoming honours thesis on the topic?