After having sex with a girl, you go into the bathroom and wipe the pussyjuice off of your dick with a washcloth. When returning to the bedroom, you throw the washcloth on the girl's face. Thus giving her a wet blanket.
Joe: "Did you see that Phillies game last night?"
Mary: "Yeah, what a mess! I couldn't believe the weather."
Joe: "I know, so disappointing. The whole city is a wet blanket now."