I asked this question last week which is related:
http://www.fluther.com/disc/23243/how-does-one-draw-boundaries-in-friendships/
It seems like being in a relationship with someone else gives you a built-in excuse. Regardless, I’d suggest being up-front about it. I wouldn’t wait until you’re out at coffee to drop the “I like you as a friend but I have a boyfriend” bomb. Marina has the right idea up above. Just clarify beforehand. You can even say something like, “I have a hard time reading people sometimes, so I apologize if this is a little out of line, but I want to make sure this is just a platonic date.”
HOWEVER, you have to also accept that if someone really, really likes you, they might agree to a “friends only” hang out while still hoping for more. Just be clear about what you want, and set boundaries if you sense that your friends are wanting more from you.
See, community? I learned!