Friends with benefits

If people are 'friends with benefits,' it means they are friends who have sex. This implies that there are no strings attached -- neither person intends to get involved romantically. They're friends, but that's it. Well, except for the sex part. The problem with this is that one person usually ends up having feelings for the other, and that usually ends things pretty quickly.

Example:
-So are you two going out, or what?
-Nah! Just friends with benefits!

What a dog!

Calling a man a 'dog' has a very different meaning than calling a woman a 'dog.' A man who is a 'dog' is being described as a person who sleeps around. A woman who is a 'dog' is being described as ugly.

Examples:
-Guys don't want a relationship. All they want is to get laid.
-Men are dogs!

-Look at this picture! Can you believe that's his girlfriend?!
-What a dog!

Get a room!

When a couple is being (overly) affectionate either physically or verbally, someone will invariably say, "Get a room!"  The room they are referring to is a hotel room, which is for the couple to have sex in.

Example:
-I love you!
-I love you more!

Person overhearing conversation -Oh, get a room!