Last night I slept with my best friend's best friend.
Why did you do this?
I just felt like sleeping with the girl
Before you do things that can affect friendships you should check with your friends. It is normal to be attracted to people.
If the situation were reversed I would be a bit jealous.
While it's fine to be attracted to people, having sexual relationships with friend's of friend's can evolve to a messy situation and it could harm your friendship. The potential for drama might not make the sexual relationship worth it.