Why is it that you can show penetration for a blowjob but not when they're fucking? That is so damn lame. Developers will take any excuse to show a dude's dick but when it comes to the woman's pussy, only time they ever seem to want to show that is if it's shaved or bald.
I think you may have overlooked that it's much harder to get the angle and things right with sex (based on how poorly many devs do that), so probably easier to hide. I agree it's a weak ass situation when the dev doesn't show it, but many of them can't get the angles right, they can't get the dick motion right, pull out too far, etc.
Anytime a dev does sex without actually showing it, you can be guaranteed that they decided to cut-time out of the rendering, posing, etc. basically they skipped spending the time needed to make a good scene.