Here's what I would do if I were you: Tell your BF about the guy who has a crush on you. Learn from your BF what his limits are, hopefully they will be reasonable. Regularly remind the guy from work that you have a BF whenever an action of his feels too personal. Don't be afraid to express anger when he flirts towards you.

You're not a bad girlfriend for seeking friends. I have been in a lot of situations just like this mostly at university, I think guys are a lot worse at this whole platonic friendship thing. Otherwise, I understand that you want to keep an entertaining person you can connect with as a good friend. I hate losing friends because I just turned them down on dating. It's like, thanks, I thought we liked each other! And then they don't give a crap about you when you won't date them.