The most common theme I've noticed in futa on male stories is definitely the power reversal trope. It's not just about the physical act; there's a whole narrative built around a dominant woman with that specific anatomy asserting control over a man, which flips the usual script. You see a lot of scenarios where a previously submissive or ordinary guy gets completely overwhelmed by her confidence and, well, her equipment. It plays into fantasies of being desired so intensely that traditional roles don't apply anymore.
Another huge one is the 'first time' or discovery angle, both for the futa character and the male character. Stories often focus on the shock, curiosity, and eventual surrender as he experiences something entirely new. The emotional arc is less about romance and more about raw fascination and taboo-breaking thrill. I've read a few where the guy is initially resistant or confused, but the sheer novelty and her insistence break down his hesitation, which seems to be a big draw for readers who enjoy that push-and-pull tension.
Body worship and size difference themes are also massive. The descriptions often get very detailed about the contrast between bodies, emphasizing the futa character's size and strength against the male's. It's a visual and sensory fantasy that translates into written form through extensive focus on reactions, sensations, and the sheer physicality of the encounter. The appeal lies in the intensity and the almost impossible scale of it, making it feel larger than life.