I doubt that you're intimidating to guys. While I hear women say this I don't truly think its an actual occurrence nine times out of ten. I think what happens is that your "snooty" look tends to come across as "I'm not open for business so don't even think about speaking to me or else I'll bit off our head and your little dog too."
Guys can sniff that out in a heartbeat. Seeing as rejection is a very real thing, many fellows will attempt to forego rejection by sizing up the various women around hoping that they'll come across one who looks inviting. That's who we go for. Nobody likes rejection.
In terms of guys you like and clamming up; while I understand that happens, you might need to realize that it doesn't intimidate men, it is an outright indicator that we should just leave you be. Not that you have to keep a silly grin on your face or totally fake the funk, but just like with anything, a person that looks scary is going to get the least amount of interaction.
So smile more. Or something.