i dont know what to do.

i like this guy... we use to just have sex because we were close and felt like we could handle doing just that. but of course here i am in tears. because with no warning he now has a girl sleeping over. i met her at his house during thanksgiving...and.... now i just don't know what to do. do i tell him i don't want to continue this abd break it off with out explanation. or do i ask him why then make a decision?