The sex in the car was consensual. Even when she knew his condom was no good, she said something like WTH. In today’s culture that means YOLO.
And she spent a good portion of the movie thinking she was pregnant, rejecting his offer to step in and do the right thing and marry her and she tells him it isn’t even his.
You're talking about Kenickie and Rizzo. The scene in question is with Danny Zucco and Sandy Dumbrowski/Olsson.
In the latter scene, Danny puts the moves on Sandy, who rejects him and storms out of the car.
Song: All Alone in the Drive-In Movie (Broadway version)
When there's one minute to go
'Till the lights go down low,
I'll be holding the speaker knobs
Missing you so-o-o!
-PJ