Women are as much each other’s best friends as they are worst enemies. From Van Morrison’s Wild Nights, “All thr girls walk by, dressed up for each other.” Women are CONSTANTLY checking out other women as competition, and thus they raise the bar for each other. This is because (evolutionarily) guys are supposed to make the first move. They don’t have to necessarily be as hot, because they can prove themselves to the female in other ways (lift heavy things or win competitions in front of her [physically superior], spend money for her/on her [good provider], pretty smile [good genes], whatever). So the more attractive the female, the larger the pool of mates from which she can choose. In the animal kingdom as well as at the bar/club scene, 90% of the time teh females get the final say in whether a pair will get together (mainly because men evolutionarily are not as picky, and will often hit on several girls in one night [Except for Jamey. That dude KNOWS what he wants]).

Here’s the kicker. While some women get defensive about other attractive women limiting their mate-pool (and usually compensate by either getting catty or sluting it up), others are attracted by how good she looks, in an “I hope I look that good” kind of way, wanting to be wanted by someone who knows what being HOT is all about.

Now, why do guys like it? Because (following the raising the bar/High standards rule outlined above) “If that girl is hot enough for that other girl, she’s probably hot enough for me.” Also, we don’t always like PDA, but if she’s down to make out in front of people, maybe she/they’ll make out with me (similar to your point 2).

Right? Right?

It could have something to do with the inherent desire of males to sow their wild oats. With two (attractive) women getting intimate with each other, an on-looking (heterosexual) male would likely want to join in on the fun, and it likely stems from males’ genetic proclivity to procreate.