Good question. A gay mate of mine tried to explain his reasoning behind it and I guess it made sense.
He doesn't have a lisp......or not really. Sometimes but mostly not.
He reckons it's all down to a lack of identity. You know you're different from an early age but you're fecked if you're going to come out. Imagine the bullying that would be dished out. So you live a life of pretend. Finally when you get that bit older, stronger and braver you come out and a rainbow coloured carpet is rolled out in front of you that allows you to finally embrace who you are with all your gay, straight and fag hag mates in full support of you.
The problem however is that you've been living a lie for so long that you really do lack an identity when you finally start living the life you feel you should always have lived. It makes sense too. Think about it. How many years do you live trying to find yourself. You really only come into your own in your 30s as far as I can see.
Imagine for gay men never being allowed to try and be themselves. It's no mean feat coming out and finding yourself and it's something that demands a huge amount of respect.
So my pal reckons that with this gross lack of identity and the welcoming open arms of the gay community, many young gay men take on the identity of their peers, who would have taken on the identity of their peers. So a lot of the gay-isms are handed down. It's why the whole gay scene is an actual culture. It's not just a scene. They are as much cultured in their own ways and "isms" as you might say people from Manchester are. The Mancs have isms that are unique, and because the whole gay scene erupted out of the underground and was widely unaccepted it makes complete sense that it was forced to live somewhat separately to mainstream society - hence it taking on it's own culture
Anyway - that's my take on it