For me, I'd prefer someone as famous as possible. I think it's fun to think of scenarios where an A-lister could almost, sort of justify to themselves that they should do an explicit scene.
One scenario I could see is a real Hollywood couple starring opposite each other in an edgy drama where they agree to have sex on camera. Maybe they decide to actually have sex for real on the condition that the penetration is not shown explicitly. The Getaway starring Alec Baldwin and Kim Basinger is a missed opportunity here. It was a sexy and fairly graphic scene (especially the cunnilingus) but could have gone further. Gaspar Noe actually asked Vincent Cassel and Monica Bellucci to star in a movie together with explicit sex scenes. They strongly considered it. Would have been amazing to see!
SCRIPT UNDER DEVELOPMENT
Ryan Gosling is a thief and Eva Mendes is a cop. She is solving the case, knows he did it but can't prove it yet. He is a perfect thief, never makes mistakes, but his attraction to her makes him sloppy. (think of the movie Out of Sight). They meet, flirt. There is strong sexual tension coupled with the suspense of the investigation and the forbidden nature of their relationship. They both give into their desires and there is a very passionate love scene. Ryan is shown going down on Eva, shot looking down her body. His lips and tongue are unmistakably on her pussy although the angle blocks any explicit view. They switch to missionary and he inserts himself. She is shown completely full frontal and the base of his erect shaft is visible in the shadows as he thrusts in and out of her. Then she turns him over and gets on top to straddle him. The camera shows her bare ass riding him with his balls and the base of his shaft visible. Then it pans around to see her in all her glory from the front, including a look a her public mound and maybe a shadowy hint of her lips. Her arousal is real, her nipples are hard and standing proudly on her chest, she is breathing heavily and her face is flushed as she approaches a real orgasm. Ryan grabs Eva's hips and thrusts up into her as she climaxes. He comes inside her as they finish together.