I also think the joke is in the eye of the beholder - I read it as a joking "i'll leave you two kids alone..."

I mean this Dean. I think within the context of his character it was said in the same light hearted way he'd talk about Sam and porn. And remember also that Cas' sexuality is portrayed as naive and almost child like. And that Meg did originally initiate the contact with Cas.

Not disputing that there are different ways of reading it, but I get tired of people saying "I read it this way therefore thats the fact"