It was initially just a one-time event while we were drunk.  Kept going because the sex was so good, and turned into free use because we both liked the scenario and have a huge Sex Drive. When he requested to bring a friend around one day, I said, "Sure, more dick for me."

He came over more often, even for drive-by blowjobs, and we had a great threesome. My roommate asked about two more buddies if they can join, so I replied, "As long as he knows and trusts them, I'm fine with whoever he brings over."

Since then, he has told all of his close friends that they are welcome to come over and do me whenever they want, whether they need a blowjob or a fuck. They are all so incredibly lustful that I get fucked several times a day, hehe. In order to make it easy to fuck me wherever and whenever, I also just run around naked.