<human>: I currently live with my friend [20M], called K, in a semi permanent roommate situation where I currently stay on his couch while we find a bigger place, this has been going on for a month. I split up with my ex [19M], called S, around a year ago after a two year relationship but we have been having sex on and off for the past six months. I have been friends with K for 5 years and K has been friends with S for ~12 years.

Since I am living at Ks house and S lives with his boyfriend (I know this is sounding worse and worse) the only place for us to have sex is at Ks house. We have had sex around 3 times since I moved in, all when K has gone to bed. 

Last night K and I had a few friends over and it came up in the discussion that this had been going on. His face immediately dropped and he looked quite pissed off. After our friends left he said he couldn't believe I had done this and that S was not welcome in the house anymore. I understand to a point why he is pissed but I also believe I have a right to have a sexual relationship in this house. It is his house on paper but I pay rent+half the bills for the right to sleep on a couch. 

Can anyone please shed some light on this situation and help me out. I don't want to lose a friend by overstepping boundaries that I didn't know existed.

Could you distill the essence of the text into 1-3 sentences?
<bot>: Had sex with ex [19M] at our mutual friends [20M] house, friend found out, got pissed and said my ex isn't allowed here anymore.