Mom/Sister/Daughter asks, in passing, “Have you gotten your girlfriend a birthday/valentine’s present yet?” He hasn’t. Girl suggests lingerie. He doesn’t know what kind to get her.
This leads the girl to model for him some different kinds of lingerie, from the least provocative to the skimpiest. She can matter-of-factly point out advantages of each: “It shows off her cleavage”, “You can see her nipples/pussy right through it”, “It shows off her ass.” Maybe crotchless panties: “You can fuck her without even taking them off!”
Eventually, she notices his erection and points it out. “It’s okay, your girlfriend would want to see it. Go on, show it to me!” [Okay, that dialogue needs work.]
At this point, the video can devolve into JOI. Alternatively, if it’s a sex scene, “Do you want to see how they feel? Go on, you can touch me!” She grabs his hand and encourages him to grope her, then lets his hand roam freely. When he gets to her crotch, she is affected.
She asks (about his cock) “Do you want me to help you with that?”
This can just as easily be a POV video that ends up in JOI, or it can have a male performer for a sex scene.
Obviously, lingerie is a kink, and this exploits it. You have choices about what the viewer sees while she is changing.