Where was the "NO" or the "Stop it" in all of this? and I hear what the poster above is saying but this could have all been avoided when he put his hands in your pants (At which point according to your post) you were awake, and on the floor with a group of your house mates watching a movie
You could have just got up and moved at that point or told him to stop, shouted hey leave my arse alone.
But no, you grabbed his dick instead. This sent a very clear signal that you were ok with what was occuring
The fact that you let it get that far says to me you wanted it to happen, and yes, that means you cheated on your boyfriend
This sounds so much like a thread few weeks back "He raped me" where some chick had anal sex in the shower with a room mate, which ended up not being a rape at all but mutual sex but she regretted it afterwards so claimed she was raped.