Bob and Fred were looking past them at something else.
It looks like Alice is seducing Diana but it turns out to be a prank to trick Fred.
Bob isn't particularly turned on by lesbians or girl-on-girl action.
Bob and Fred were looking past them at Robin and Allison.
But as soon as Fred leaves then Alice and Diana are having sex.
At least, that's what he claims; he later makes his excuses to have some,ahem, alone time.
Alice and Diana have to shoo away the people with cameraphones if they want to make out.
Alice and Diana are really, really, ugly, but people still treat it as the hottest thing ever.
Alice and Diana are known to be straight. They accidentally brush against each other momentarily. Heads turn immediately, and comments of this sort are made.
Alice hugs Diana after she nearly fell over. Everybody around them notices. The entire world stops and takes notice, causing a Rube Goldberg Device set of events which ends in a nuclear missile being launched.
Because everyone thinks girl on girl is hot, Alice is afraid to come out for fear that everyone will just think she's looking for attention.
It's not rape, it's surprise sex or role playing.
Fred introduces Bob to a pair of Lesbian Exhibitionists.
Lesbians that aren't ugly are introduced.
Plotted A Good Waste:
After getting fed up with people leering at them, Alice and Diana put on a deliberate show to draw in as many of those people as possible, then deliberately move into carefully orchestrated Squick territory so that the onlookers are completely grossed out and it's their own fault. May or may not be one person who finds what they saw to be even hotter than the normal makeouts.