What did Dean mean by "It's an federal thing" ?

[Jamie Turner]

In the Supernatural episode Everybody Hates Hitler, Dean gets flirted on by a guy. He keeps pointing at himself saying, it's a federal thing, it's a federal thing. What does this mean.
Ps- Dean is obviously gay. Especially for Cas!

[Katrina E.]

Dean was wondering why the guy was following him and so pulled his fake FBI badge and asked him. When the guy implied that they had a "thing" back in the quad, Dean gets flustered and says it's not a thing, it's a federal investigation and then later a "federal thing" referring back to it being a federal investigation.

From the scene:

DEAN - Special Agent Bolan.

DEAN puts his badge on the table and the BEARDED YOUNG MAN looks at it.

BEARDED YOUNG MAN - Oh, really? Wow. I thought you were like a headhunter or something. [He laughs.]

DEAN - This is the second, maybe third time I'm seeing you today? Why you following me, Gingerbread?

BEARDED YOUNG MAN - Oh, so we, um... we didn't have a thing back there, huh?

DEAN - Back where? W-what, now?

BEARDED YOUNG MAN - I'm sorry, man. I – I thought – I thought we had a thing back at the quad, you know – a little "eye magic" moment, and I saw you here [DEAN picks up his badge] and I figured I'd wait until you were done with your meeting and then maybe we might, uh...

DEAN - Yeah. Uh, okay, but no – uh, no moment. This is a... federal investigation.

BEARDED YOUNG MAN - Is that supposed to make you less interesting? No. I – I'm sorry, man. I hope – I hope I didn't freak you out or anything.

DEAN - No. No. I – I'm n-not freaked out. It's just a, you know... a federal thing. It's, uh... Okay… [his phone rings] citizen. As you were.