A steady source of sex partners. Usually diverse in geography, it's made up of people willing to have sex with no emotional ties. Works best when they don't know each other.
Is that one of his fuck buddies? Oh yea, he has so many women in the Pipeline most of time his phone rings its a booty call.
Wow! I'm having a hard time managing the Pipeline. Can I send some your way?
Did he fuck her? No, she wasn't ready. So he put her in the Pipeline. (ie. He "pipelined" her)