[Autocratic]

I'm pretty baffled that some of you think this is OP's fault. You didn't knock, that was your only mistake. But they are in the wrong for the following:

A. Getting it on in your bathroom (unless you have an agreement in which this is deemed acceptable). No one looks for contact lenses naked, you should immediately call him on this blatant lie.
B. Not locking the door when getting it on in your bathroom.
C. Yelling at you after the results of A and B played out.

Your roommate is likely embarassed for acting like a douche bag and tried to make up for it by acting as if it were your fault. It's not. You don't have to be pissed at him, but let him know that you believe you are not in the wrong (and absolutely don't apologize).