I'm sorry but no matter what, if you consensually let a man fuck you, or fuck a man. You are gay, or bi. what you described is being bi.
with "heteroflexible" do you mean bisexuality? because fucking a guy is literal definition of being gay. I am bi/gay myself, but the idea that you could have sex with a man, even if it was one time, and still be straight is flawed as that makes no sense.
It reminds me of this scene in a movie where a black man was claiming to be fucking men, but since he has a wife and kids, he is straight