I don't think there is a clear cut-off line. Heteroflexible is still Bisexual, the difference being one of frequency--how much homosexual activity do you have?
My interpretation of heteroflexible was someone who was hetero, but tolerant and understanding of anything else, as well as possibly open to activity involving the same sex, that either hasn't had it yet, or may have had a close encounter, etc.
I think when there is a conversion from straight to bisexual, there is a bi curious intermediate step. A straight person will know when s/he has become bi curious because obvious behaviors will dictate that. But there is a transition period to becoming bisexual, and it's bi curious, for all intents and purposes.
In the bi curious phase, a person who has been straight still doesn't know for sure if s/he likes same sex experiences, and is experimenting. Some may turn back around and decide that same sex encounters are not for them, and they will go back to being straight or remain no more than bi curious for any given period thereafter (I know there are folks on here with strong opinions who will disagree with this, but there it is. )
In my own personal experience, I knew the moment I had made the transition from bi curious to bisexual. It wasn't until after my 1st few encounters, because during those first few encounters, I still wasn't sure if it was a lifestyle for me. There came a magic point where I knew what I wanted and how I wanted to go about it.
When I was straight, I would not have wanted a male play doctor. I saw to it that all of my real doctors and specialists were women, as much as possible.