I understand, my relationship went downhill because of it, most guys are open to it because they want a threesome and I didn't, I wanted a woman all to myself:P
It was hard and well it became a story of insecurities which he later took out on me and well I guess Im not longer with my hubby but I feel relieved, I realized I was that much better off.
So in regards to advice, just explain that you are attracted to woman, although if your not for threesomes, also express that out loud!!!!!
Good luck if you havent done it yet?