Darwin missed some stuff in his research. We still largely follow his model and use lots of his terminology even now that we know a bit more about the topic. We still use his name when talking about the field of study that he pioneered - even when we're talking about concepts that he really didn't address.

The cultural bias argument contends that there is immense pressure in many of the current dominant social and religious cultures to not engage in male/male sexual activity. Kinsey's scale explanation (and the core of his research) remains incredibly significant to the conversation - but arguing that someone isn't bisexual because they're long term monogamous for example is clearly fallacious if that person is sexually attracted to people of both genders, but chooses not to act on that attraction because they value their relationship agreement to be monogamous.

The reality of our modern understanding of sexual identity is that attraction is definitive far more than specific action. We know there are many, many women in particular who are long term exclusive heterosexual in terms of activities engaged in but who have had extensive relationships or sexual activity with same sex partners. And that there are equally substantial flaws with Kinsey's activity oriented weighting with many other demographic groups.

As an example of the flaws of Kinsey's weighting, my dad is lifelong identified as homosexual but engaged in heterosexual activity with my mother to have me as part of a deliberate and mutually agreed desire to have a child together. My dad hasn't had sex with a woman since they found out that my mother had conceived. They tracked exactly when she was ovulating and had just enough sex, just enough times to get a kid. But at the time? He was only banging my mother while they tried to conceive, and not any dudes. It would be ludicrous to call my dad anything but homosexual - but if he'd been assessed by Kinsey's research, using Kinsey's weighting while they were trying to conceive - he would have showed up as predominantly or exclusively heterosexual.