It’s got to be Grease:

Sandy is a sweet lovable young woman who prior to the start of the movie had a summer romance with Danny. But now that summer is over, how will they get together again?

The solution to this problem is for Sandy to tart up, slap on a ton of makeup, wear a slinky outfit, and start smoking. Because that good girl she was before just wasn’t enough to satisfy he-man Danny. Now that she’s a slut, hey, no problem.

I’m not a prude when it comes to romantic movies. I would even be fine with Sandy hopping into bed with Danny as long as it was mutually consensual and honest. But the transition of Sandy from a wholesome teenager to what is basically a prostitute just seemed like something the character would never do. And for Danny to be thrilled by this complete turnaround of her character seemed equally forced. This is what held him back from committing to her? The entire reason he was attracted to her was that she was different from the women he usually encountered in his seedy circles. Now she’s just the same as them, he becomes more attracted to her? It doesn’t sell for me.

It just left a bad taste after what was a fun movie up until then.