Is this all just speculation because you've found yourself attracted to this other man, or has your coworker expressed an interest, like flirting with you, or asking you out? Does he know you're into him? Or did you already start to date him? His part/participation is unclear.
As to what you should be aware of and what you should do, basically, be honest - with yourself and these two men. Stay aware of your feelings and if it doesn't feel right, don't barrel over your own emotional life. There is nothing wrong with saying, "I've never dated two guys before, and I don't know what I'm doing."
Also, ask this guy what his expectations are, and how he incorporates poly into his life. Ask a shitload of questions. Does he have rules or boundaries in his other relationship(s) that would affect you? And figure out what your own personal boundaries are. What do you need to feel respected, appreciated, and whatever else you may be looking for out of this? Talk to your fiance about what safer sex means to the two of you and know what you can do. Then assess whether or not it's a good fit for you, before jumping into the sack with him. Keep your eyes and heart open and your feet on the ground, and you'll be fine.
Last edited by nycindie; 11-22-2012 at 01:58 AM.