Mine aren't dirty or kinky, they are just impossible.
But my fantasies usually revolved around growth. so watching a girl grow taller or her breasts grow, or me suddenly growing taller and more muscular and my genitals growing. borderlines giantess/giant fantasies.
more childish some think, but whatever, it's what I like.
it's not very childish and far more common than people think.
I actually have similar fantasies but Female Muscle Growth aligned than height growth.
and also fantasise about my penis growing to pretty insane proportions.
luckily I do some online roleplaying with some female friends with similar interests, so it works out well for me.
as for other sexual fantasies: I don't really have anything that can be considered seriously dirty or kinky.
I'm pretty vanilla in those regards.
but do have fantasies about worshipping/making love to a female bodybuilder, being with a busty mature Asian (preferably Japanese or Chinese) woman and your basic F/F/M threesome stuff.
also have some mild mental domination fantasies but they are pretty tame compared to other stuff and what I used to get paid to do years back.