the definition of "bi" was simple when people were less aware of gender as a social issue it was simply "they like boys and girls” but as gender identity has really opened up it’s not so clear cut

(not to imply that nonbinary identities are an exclusively recent thing -- more saying that my awareness has expanded and nonbinary identities have become much more widespread lately)

now the definition of “bi” is all over the place,like you said. some define it as “men and women,” some “men and women, no trans allowed,” “same gender + other genders” is a common one, “attracted to two particular genders” is a very literal approach that some people take
i can’t even put a finger on the most common definition (outside of more LGBT-positive spaces like Tumblr i think it’d be “men and women,” but it’s hard to say around these parts)
i’m sure there’s a lot to say about the relationship between sexuality and gender -- like, i’m a trans woman who’s pre-everything -- hormones, surgery, etc. i wouldn’t expect or insist that a lesbian or straight man express some kind of attraction to me on gender alone, especially when i don’t present “right”

but what’s sexuality about, then? presentation, genitalia?

most exclusively heterosexual or homosexual binary individuals can look at someone with clothes on, not seeing their genitalia, and determine whether or not they're attracted to them. some would revoke this attraction if they found the person's genitalia didn't match their preferences. some wouldn't. some would revoke this attraction if they found out that said person's gender identity didn't match their preferences. some wouldn't.

idk, how can we define sexuality in a way that's accurate?