As a person who is currently fooling around with Koikatsu, I can tell you already animations are very, very time consuming, even with already pre-made animations used. It takes quite a lot of time to set everything up and change to your liking. And there is way too many things to take into consideration.
Like, for example, I'm using my tall character and I want it to have an intercourse with average height, yer rather pettie, girl. So I'm picking pre-made pose, setting up camera angle and... I can't call it done, becuase I can clearly see there are errors, so I'm playing the animation and I'm looking for those things. Like hands placed in the air instad of buttocks or on the calves instead of knees, dick clipping through the girl at the end of the movement upward. So now, fixing. Moving the hands to thier right spots, decreasing the size of dick, a bit. Now it should work, right? WRONG! Dick is going out of the girl at the end of movement backwards. And those hands when placed on the buttocks are now a bit to long. So maybe different camera angle? But I need to see her face! Damn it! Now I see MC's face in that angle and I don't want to!
...
Fix it till perfection.
...
Wait, why it's 4:00 in the morning? When I started it was 21:30!
With such excessive use of his dick it's understandable. Those girls are grinding it faster than MC can regenerate!
Why? I want to be able to compare myself and be bigger... at least once.