Create the plumpest personal pillows you can, and really push it.
Now copy the model and make the sort of flat ass homophobic straight men would sculpt / tell artists to change.
Once you are done, bake in the value and reimport to optimise, or leave it and add it in as a fun easter egg or unlockable.
To get the ideal values for a local area, (like a man's glute-a-toots) mask the bum using vertex paint (say, red for that spanked look) and in the shader...
Now you can ramp that up and down and you have a plumpener value. Again, you can bake this.
Ass perfection in real time? What is this sorcery.
To scrunch that man muscle right, you should think about how the underlying muscles work.
That requires your pipeline to let you tweak in engine and have a fast turn a round.
That's it, obviously I need coffee.