Okay, first off, full disclaimer. I'm a happily married straight female.
That said: I think there should be MORE gay PDA. I have a very good reason for this.
Out of all the small children who will see two girls or two boys kissing there will be some who will grow up to be GLBT and all the others will know someone who is GLBT. If kids (and everyone for that matter) are exposed to affectionate gay couples and it is considered just as normal for two men or two women to smooch in public as it is for a man and a woman to smooch in public, those kids will internalize that perception. If those kids internalize that perception those kids will either grow up to be non-homophobic or may just be more accepting of themselves when they become teenagers and suddenly (gasp!) understand in a cogent manner that they are attracted to people of the same sex.
As a former HS teacher, I dream of the day when gay teenagers have the same percentages of suicides and depression as their straight peers, and I honestly think that having same sex PDA accepted on the same standing as straight PDA, as well as knowing family, friends, neighbors, teachers, ministers, cashiers, shoe salesmen, accountants and fire fighters who are somewhere in the GLBT spectrum is the only way that will happen.