You Know How The Whole "Know How I Know You're Gay?" Thing Started...?

From "The 40-Year-Old Virgin":

(Cal) So you're gay, now?
             
(David) No, I'm not gay. I'm just celibate.

(Cal) I think... I mean, that sounds gay.
I just want you to know that this is the first conversation of three conversations
that leads to you being gay. There's this... and then in a year it's like, "You
know... I'm kind of going to wanna get back out there, but I think I like guys."
And then there's the big, "I'm a gay guy now."

(David) You're gay for saying that.

(Cal) I'm gay for saying that? 

(David) You know how I know you're gay?

(Cal) How? How do you know I'm gay?
             
(David) 'Cause you macraméd yourself a pair of jean shorts.

(Cal) You know how I know you're gay? You just told me you're not sleeping with
women anymore.
             
(David) You know how I know you're gay?

(Cal) How? 'Cause you're gay and you can tell who other gay people are?

(David) Do you know how I know you're gay?

(Cal) How?
             
(David) You like Coldplay.

You know how I know that you're gay?

(Cal) How?

(David) You like the movie "Maid in Manhattan".

(Cal) You know how I know *you're* gay?

(David) How?

(Cal) I saw you make a spinach dip in a loaf of sour dough bread once.

(David) You know how I know that you're gay?

(Cal) How?

(David) You have a rainbow bumpersticker on your car that says "I love it
when *balls* are in my face".

(Cal) That's *gay*?