i'd have to venture to say that it's because they've been confronted face to face with a solid issue.. and after looking back to their years of elementary school.. and noticing that they really didn't have a lisp back then.. they've been faced with the fact that they really have acquired a lisp over the years... perhaps to express their homosexuality.. for all to see.. or maybe as a form of mating call.. so as to let other gays in the area know that they themselves are in fact gay.. but either way.. they're just plain old embarrassed for some reason.. perhaps because they never really thought of themselves as having a lisp.. or never faced the fact that they did... and now that they look at themselves deeply.. they are realizing.. "oh my god, i'm just like all those other flaming gays that talks with a lisp so as to revel in my homosexuality and all its glory"..
who knows..
all i know is.. you're right.. and i'd say that 70% of the gay men i've met/known have had SOME degree of lisp.. however light.. there was definitely something there..
I think this is a good question, although gay guys won't admit it.
I wouldn't call it a lisp though, more of a half feminine voice. Sometimes you cant tell if someone's gay...and then they talk and all is revealed. Its quite funny really.