This is an incredibly damaging, old fashioned and frankly bigoted stance. You don't "decide" your sexuality, it is inherent. Using language like "experimenting" or "phase" completely invalidates the feelings and experiences of young people and you run the risk of alienating your daughter and damaging your relationship.
It is possible to be confused about your sexuality and need a bit of experimentation before properly concluding your orientation though!
It’s definitely not immediately obvious for some of us (women supposedly experience this more than men, statistically, but it’s hard to be sure of that because comphet seems to be part of female socialisation?).
All my female friends (including me) had short relationships and/or intimate experiences with other women in the 90s.
I wouldn’t completely rule out a future relationship with a woman but I think calling that teeny possibility bi is a massive stretch and likely to do a disservice to actual bisexual people who often do experience difficulties and discrimination.
There is a bit of a tendency amongst current early secondary age kids to sit under a metaphorical LGBT sorting hat.
If you are a year 7 summer baby, that could well mean picking a sexual orientation label before you’ve ever truly experienced real, loins-on-fire sexual attraction, and are still in the same sex non-sexual crush phase of life (mine was on a female maxillofacial surgeon who patched me up after an accident. Phenomenal woman, I felt like I was in love with her. I was 11 and it was actually a heady combo of absolute trust, total respect and sincere thankfulness).
I don’t really know how to navigate the current day teen obsession with labelling oneself - I’m sure lots of heterosexual parents of teens would appreciate some advice on how to respond to the coming out announcement!
My daughter rather dramatically announced herself to be bi at 12, so far we’ve gone with ‘We don’t mind who you date or who you eventually fall in love with, girl or boy, just as long as you make each other happy and treat each other with respect’, but it felt like she was almost disappointed in our non-plussedness!