Oh the wonders of 3D

I just realized that if you were skilled enough with 3D, you can go and create your fantasy girl and do watever you wan with her. Pose her nude etc etc.

Made me thinking was that if you made a 3D representation of a real girl (ie someone you had a crush with etc etc) and do watever dirty/fantasy stuff with that model for your own private entertainment. Would that be illegal?

Well, if you model someone (alive) without their consent for that kinda thing ... most likely illegal.
If you model something fictional ... like Lara Croft or whatever, most likely a trademark breach and also illegal.

Depends on the country you're in, of course.