This is an area where the genres kind of intersect with some interesting, messy real-world feelings, I think. On a basic level, you've got the physical fact of two characters equipped for penetration, which immediately scrambles a lot of conventional scripts about who's 'active' and who's 'receptive.' It's not just a simple role reversal, though. I've read stuff where the 'male' character is still written as emotionally dominant or more aggressive in pursuing the futa character, so the power play gets layered—physical capacity versus social expectation, you know?
Sometimes the desire dynamic hinges on the futa character's own conflict or duality, a tension between a softer, traditionally feminine side and this more assertive, phallic aspect of themselves. The male character's desire can then be about embracing or submitting to that totality, which is a power shift in itself. In other stories, it's framed as a mutual, almost competitive exploration of pleasure, with the power flowing back and forth. It's less about one person having it all and more about the constant, slippery negotiation of it, which honestly feels more true to how complicated actual desire is than a lot of strictly vanilla M/F stuff. The best ones make you forget to even categorize the characters and just get sucked into that push-pull.