@PonyPon  
If the statistics on gays are anywhere near right, they actually make out less often in public:  
There are way too many straight ‘Public Displays of Affections’. There should be more lesbians kissing in public, statistically speaking, if they acted like normal people who wouldn’t be discriminated against.  
But, of course, that’s just in my experience; It could be a coincidence, or I just hang out in places lesbians aren’t interested in.
 
But even outside of the discrimination, the kind of young people who do a lot of PDA usually seem like they’re doing it to ‘claim’ the other person; They want people to see them kiss, both to show off their lover and make sure everyone knows they’ve claimed them.  
It makes sense that it’s less interesting for lesbians to kiss in public; Most of the viewers will be straight, so your intended audience (gays of your own sex) is much less dense in any given place.  
Bisexual people do have plenty of reason to kiss in public, though, and the less faithful they are the bigger the benefit (and let’s face it, teenagers and barely-beyond-teenagers, on average, aren’t known for their faithfulness)  
But not gays.

“Do lesbians not make out in public alot?”  
“No, Bill, they’re people like you and me.”