It definitely wasn’t mainstream here in the Netherlands up until the mid 1990’s from my perspective. Any guy who did things with another guy was ‘gay’, irrespective of anything that they might engage in with women.
I imagine if that guy grew up in say, the 70’s UK, they genuinely might not have been aware that ‘bisexual’ was a thing you could be.
I wouldn’t be surprised if one of the factors is ‘we stopped beating kids’. Back in ye olden days, they’d hit kids until they stopped being ‘weird’. You obviously can’t do that today.