I would like to second Jane. I know that some see it as "completing the picture/puzzle" but I would be perfectly happy with any of my relationships as a two person one. That special criteria is the reason why I knew that it would work for me in the case of my husband/boyfriend. If a partner is missing "something" I will not feel the need to be with him/her. This may be different for others, as Dagferi pointed out, but not neccessarilly for all
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.