Hot sex with my girlfriend

http://tinypic.com/r/10pu6th/9
I am 20, my girlfriend is 19 and we have been together for two years. She is a great looking brunette with a fantastic figure.
I'm having amazing sex with my girlfriend's best friend... who's dating my best friend
We have always been best mates as well as lovers, though just lately she does not seem as much fun as she once was.
I knew her friend slightly but then she started dating my best mate so we all started hanging out as a foursome quite often. She is 20 too.
One night the four of us were at a party. Me and my girl’s mate are smokers so we went outside into the garden for a cigarette.