Irrational fears?

So i just found out that i get to (have to?) meet my girlfriend's dad tomorrow! And somehow I'm kind of panic stricken about it! I think it might be that she isn't out to him and so I'll be meeting him as her "best friend". Although this takes some of the pressure that comes from meeting the parents of someone when they know your dating, at the same time Im really scared of messing that up!

Will he know I'm gay? do I pretend to be straight? what if he can tell from the way I look at her/talk to her etc that we are dating?

So much pressure! and so much potential for disaster!