What's with the first two posters encouraging the guy to become a cuckholder? LunV93 who actually got herself all screwed over due to the same dilemma yet is coaching the guy to do what got her screwed over? Did you not learn ANYTHING from that bad approach?
OP: Just distance yourself from her because you're starting to crush on her and you're allowing yourself to become vulnerable to her because you find her to be "safe" because she's already taken. Stop doing one-on-one date like things with her during your work hours.
Time to step away and if she asks you why you're turning down her invitations, tell her that you find it inappropriate to be doing such tings with someone that has a boyfriend and that if she's ever single then you'd love to take her out on a real date... then distance yourself again. Keep it simple and professional from there on out.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion