Alice and Diana were looking past them at something else.
It looks like Bob is seducing Fred but it turns out to be a prank to trick Diana.
Alice isn't particularly turned on by gays or guy-on-guy action.
Alice and Diana were looking past them at Charlie and Evan.
But as soon as Diana leaves then Bob and Fred are having sex.
At least, that's what she claims; she later makes her excuses to have some,ahem, alone time.
Bob and Fred have to shoo away the people with cameraphones if they want to make out.
Fred and Bob are really, really, ugly, but people still treat it as the hottest thing ever.
Bob and Fred are known to be straight. They accidentally brush against each other momentarily. Heads turn immediately, and comments of this sort are made.
Bob hugs Fred after he 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 guy on guy is hot, Bob is afraid to come out for fear that everyone will just think he's looking for attention.
It's not rape, it's surprise sex or role playing.
Fred introduces Diana to a pair of gay exhibitionists.
Gay characters that aren't ugly are introduced.
Plotted A Good Waste:
After getting fed up with people leering at them, Bob and Fred 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.