In order to do this right, they'd have to have a system where one figure could react to the size difference of the other.
A good example is slow dancing between a male and female, in the kissing pose.
Unless the woman's ass is exactly the right size, the man's hands either float above her ass or get clipped.
First step would be making each avi reactive to the skin level of the other.
As a programmer, I can see that this may simply not be possible. It would call for very small on-the-fly adjustments in each animation. And this is just hands on skin. We aren't even talking about things that are able to change a lot more, like breasts. This is just her hips and ass. Also quite variable. But until you can get each character to react to size differences in their partner, all of this regarding more breast play or penis size changes are basically non-starters.