Basically, because Velma's not conventionally beautiful (like Daphne), is intelligent (unlike Daphne) and doesn't need constant rescuing and reassurance from a man (like Daphne)...she must be a lesbian...according to some other men. God forbid she just be shy or might actually have enough self respect to not want to put herself around or act like a bimbo for the sake of fulfilling not only a gender but also a sexuality stereotype. Another part of their argument is the fact that she is not overtly heterosexual in the cartoons, which is a completely spurious argument as Scooby Doo is aimed at children and nobody is overtly sexual in it. It would mean Daphne is also a lesbian...which could never be...she's far too pretty...
I have no idea if Velma is gay and I don't care either way but what I do care about is why it matters so much to certain people that she has to be labelled at all. Why is it so unimaginable that an attractive woman might be gay? Why must a more masculine / reserved / independent woman be hiding a sapphic secret? It makes me think of the scene from But I'm a Cheerleader where Jan realises she actually is straight and no one believes her because she's the butchest one there. It is like, because a woman makes herself unobtainable to a certain type of man, he must instantly make her unobtainable to all men so as to help his own insecurities (though please note I am only referring to a certain type of man). In the end it just becomes offencive to both straight and gay women by a) suggesting being smart is less desirable than being stupid and more conventionally attractive, and b) by suggesting than because of this she must be a lesbian.
The one thing that is for certain is that however you see Velma (lesbian, geek, introvert, know-it-all) she's still damn cool and the Scooby Gang would be a bunch of goners without her...and if I'm being honest...I think glasses and knee socks are way cute.
In fact here are some other famous real and fictional female glasses wearers to try and change your perception:
Marilyn Monroe in How to Marry a Millionaire
Enid from Ghost World
Dr Harleen Quinzel aka Harley Quinn
Hell there are loads...take a look.
And I guess because it's only fair seeing as I'm talking about girls in glasses...here I am in my new glasses, also proud to be a specky four eyes:
Maybe another time I'll do a post about men in glasses. I think they're super cool (sexy) too! Though just in case I forget...here is one of my all time fave glasses wearers (I'm sure you can't guess who...)