It's not incorrect though. People do experiment as teenagers. Some heterosexuals try being gay as teens, some gays try being straight as teens. I know three or four girls in my high school year that "experimented". All of them are now straight. The only girl from my high school year I know of who actually is a lesbian, had a boyfriend then and a couple of guys I know are still changing their minds every few months and they're young adults now.
Besides, I think that you question people's sexuality based on who they are (or more accurately who you think they are) anyway. Like I never believed that one of my highschool friends was straight - and rightly so, he's not only out-gay now but practically a gay stereotype - and very proud of the fact. At the end of the day though, whether you believe someone about their sexuality or not, it doesn't matter since arguing with them about it is completely futile and it wouldn't change your relationship with them anyway.
Besides, I think that you question people's sexuality based on who they are (or more accurately who you think they are) anyway. Like I never believed that one of my highschool friends was straight - and rightly so, he's not only out-gay now but practically a gay stereotype - and very proud of the fact. At the end of the day though, whether you believe someone about their sexuality or not, it doesn't matter since arguing with them about it is completely futile and it wouldn't change your relationship with them anyway.