Cheating always starts with the emotional, then it gets physical. Not unless its just a one night stand, then its a different story altogether. There are things that would always be unacceptable to one person and be acceptable to another. If you're not comfortable with it, and he can't respect that, then he's not a very good boyfriend now is he? Let's put it this way, what if you have sex with your friends everyday, and you tell him to get over it because its your way of expressing friendship and he has to deal with it... How would he feel if he saw you having sex with a friend or two, and then tell him you love him? would that sound convincing?
Bottom line, It's just a pathetic excuse on his part so that he can flirt around and maybe put someone on the backburner. Like you said, both of you are 20, so he's still a moron. I wouldn't expect much from him
He who laughs last, thinks the slowest