I think this can be interpreted in many different ways and should be- Rent means something different, maybe similar, but different to everyone so that scene can be taken any way it wants to be. If there is no "official" reason given, then who knows who is right and who is wrong?
As for the phone, I have never seen a phone in the broadway production and I have seen it many times...
and as for Mark being under the blanket, maybe its because they just needed him on stage lol i dont think there is really an answer to that but I think its also supossed to represent the group as a whole and their friendship and when they take off the blanket and Angel has died they are all facing away from each other and separate because they are being broken apart by the death and all the other things going on.
He is not having sex with anyone necessarily, which is why people think it is an orgy I think. Maybe there is "mucho masturbation" going on
This is not to be taken as fact, just my opinion so please don't jump down my throat. As I said before, this scene can be taken a million different ways and is just opinion here.
"That boy could use some Prozac"