Because being queer is fundamentally about sexual attraction, and if we don't touch each other then we just look like friends. Any kind of romantic or sexual touch removes the ability to pretend.
Anyone who freaks out at sexual touch (on same level as straight sexual touch in that context; I don't ask people to accept what frightens the horses) isn't really OK with homosexuality.
When it's safe, I don't combat it, I ignore it and treat my partner with a level of PDA that we find comfortable and is on a par with the straight couples around us. When it's not safe, I am uncomfortable and stiff. I'm sure that straight couples visiting places where no male-female PDA is socially acceptable feel the same way.
Rampant? Depends on where you are and who you're with.