Deep End (1970)

The blonde has long hair and a slim build, wearing a tight top that shows off her small tits. She’s sitting next to the brunette who’s got shorter hair and a similar frame, both watching a movie with a sexual scene playing on the theater screen. Things escalate quickly — no teasing — the brunette turns and starts kissing the blonde, hands going straight for skin. The blonde leans back, letting the other girl dominate, then switches it up by straddling her in the seat and grinding hard. You see full frontal during the transition, not just flashes — the lighting from the screen makes it moody but clear. The guy sitting behind them is just watching, no involvement, completely passive as the two girls go at it under the dim glow. Camera stays at a medium shot from behind, doesn’t zoom in but frames the whole encounter naturally, like you’re in the theater too.