I can only speak as a straight female, but I can understand your colleague’s position. I have many male friends in whom I have no sexual interest, even though they are attractive and we get along very well. I also love my male friends but don’t consider each of them a potential lover. I would have to conclude that a lesbian would differentiate between a friend and a potential lover the same as a straight women would make the same distinction about someone of the opposite sex. Why not just enjoy your friendship with this woman without worrying about sex, the same as I enjoy my friendships with my male friends without worrying about sex?