"Unfortunately, this reveals that teens may overestimate the safety of using condoms most of the time with a casual partner and underestimate the risk of unprotected sex with a serious partner,"

Please tell me I'm just reading this wrong and she didn't just say that teens overestimate the need to use condoms with a casual partner?

I found the wording of that confusing, too. Maybe she means they're more likely to think that as long as they use a condom they can have sex with whomever they want and nothing bad will happen? But she still seemed to be saying that they mostly don't use condoms with casual partners.

I think it means that they overestimate the safety of using condoms MOST of the time, not ALL of the time.

"Eh, I've used condoms with everyone else and I'm too drunk to get one right now... I'll be fine."

It was confusing.