I think this is a very male driven occurrence. Think about it. Apparently a lot of guys would like to have sex with two women. A lot of women are bi curious (so are guys but they hide it). Society and your best friend seem to accept women on women sex so that's a much easier sell.
Having a unicorn is a situation where much of the male ego is not threatened. There is no penis to contend with and therefore there is less competition and more bragging rights. If I pull up next to another cruiser motorcycle I feel competitive; who will get to the next light first, who's faster? But when I pull up next to a crotch rocket I feel no competition at all; it's faster almost without a doubt. The crotch rocket is essentially a different genitalia in it's own class with it's own type of energy. Therefore I don't mind telling my buddies how I sped away from me....they get it.
Playing the Game of Life with Monopoly rules.
Monogamy might just be in my genes
Poly Events All Over