I think, orangeskittles, what you said about the "reminding him of April" works, especially in the show, more so than the movie. I mean, Mark even says that Roger found the note from April, after she had already killed herself (at least that's my interpretation). So Roger probably blames himself for not being there for her, hence all of his protectiveness over Mimi with the drug dealer.
Also, Emcee is right, Roger is really nervous and awkward. This is the first girl in his life since his tragic relationship with April. Does he really want to start this again? Heck, I wouldn't feel all that comfortable if a person with Mimi-like traits started coming on to me, especially if I had an experience like Roger had with April!
Q: What is the most weirdest or funniest thing a fan has asked you?
Joe Flanigan, Stargate Atlantis: When a fan asked me for help with his grammar. I'm available.