OK, neck on the line time, please do not hate on me as I'm being 110% honest about my beliefs and I am ALWAYS open to challenge.
I have always used the example of a man, married to a man, who's fucking women and getting his kink on with women on the side then coming home to his husband (whether the husband knows about his jaunts or not).
Introduce this fictional couple/ individual to me and I will believe there is such a thing as "true" bisexuality. Experiences validating this, FOR ME, are:
1. I'm afraid I've met far too many gay guys, since school age (most of the "bi" guys I've known) and including myself, who came out as bisexual to test the waters, for themelves as much as their peers probably, i.e. convinced themselves they were 50% "normal" before becoming fully comfortable with the fact that they're attracted to men and becoming 100% gay.
2. I've experienced/ heard/ read/ become aware of a plethora of situations where a man is married to a woman, fucks around with men and the wife has no idea. Never the opposite.
Look, no hate, if you wanna tell me you're bisexual or straight and you can actually carry it off (I have internalized homophobia so am super attracted to "straighyt" guys" mainly), I'm gonna be down on my knees for you regardless and no mistake. The illusion will blur if you get on your knees and especially if you beg for my big todger up you but regardless..
NB , this is with kinks, exploration, curiosity, etc. aside. At one point in my life I did manage to feel a fanny and my also-bi mate John (now 100% homo unless you count when he's "Lola" at the weekends and ostensibly straight?) was the class heterosexual whore so go figure...
Come at me, bring it on, change my mind, please! ESPECIALLY if you can honestly provide that rare example or marriage (and not including fiction ... they actually did that on Fall of The House of Usher on Netflix which, out of context, I totally recommend watching it's farkin excaellent!