Almost any straight man fantasizes about watching a pair of lesbians coupling, but beyond watching, there really isn’t any realistic way to imagine himself in the situation without turning it into a F-F-M threesome, which I think people believe feels less intimate than a pairing. So in this regard, a lesbian pair where one woman is futa is a way for a man to better empathize with a lesbian coupling. Although a futanari character has both parts, she is still considered a woman and thus her having male parts doesn’t spoil the inherent lesbian purity of the encounter, much in the same way that many lesbian couples have no problems with using phallic toys.