Sexual union in a dream may mean that you are in the process of integrating some new behavior or aspect to your personality. Its a process of growth that goes on more or less all the time, but only occasionally erupts into one's remembered dreams. The personality trait or behavior is masculine. Think assertiveness, initiative, action-oriented or rational thought. The images in your dreams are of your fiance and of some stranger. It may be that some of the aspects are also aspects of your fiance's personality, but other of the aspects are not his and so were projected onto a "stranger". It may be not so much that you "desire other men" or "will be unfaithful" but that you are becoming more comfortable in expressing your sexual appetites - taking the initiative which the man stereotypically does. Please also see: Orgy