I don't know about unpopular, but I'd say mine is unrealistic.
My fantasy would be to get strapped into a diaper with a futanari woman. She'd be bigger than me, both height-wise and cock-wise, and she'd remind me of both while we took turns fucking each other in the diaper. When I fucked her in her pussy, She'd laugh a bit, say things like "is it actually in?" or "is that all?", but her flushed face and stifled moans show she's not that good at pretending. It could just be the bit of oral pleasure I might give her cock tip sometimes, but that just seems to make her lose composure a bit more, and she'll still be kind of hot and bothered without it.
Once it was her turn, she'd ram my ass like there was no tomorrow. During this, we might make out, I might drink from her breasts, a bit of both. One way or another, her milk ends up inside me, and much more than could be realistically expected, or handled by my system.
Her milk is rich, and agitates the stomach in large amounts, and the sheer size of her cumload might as well be an enema. Couple that with how much she streched my asshole, and I inevitably make a huge mess in our diaper.
Not wanting to be outdone, she brings her breasts up one at a time, drinks from herself, and makes an even bigger mess than I did.
And there we are, full bellies and a fuller diaper, and we would just cuddle or frot with each other in the afterglow.