Even this would be criticised by some people. i.e. the idea that things like gender and sexuality are locked-in at birth and not fluid. But the opposite would also offend people (i.e. that gayness is nurture, not nature). i.e. you can't please anyone ever so why bother? :)
I don't agree with you. (For now, let's ignore the fact that there's an unequal weighting applied to women than men.). The way the code is now means that sexual preference is not baked into each pawn, and that bisexuality is expressed only via active bisexual relationships. A pawn does not remember if it is bisexual or not. That code is applied to all pawns, which means that x% of a pawn population will be bisexual without explicitly having to track which pawns are bisexual or not. It's an effective way to model it. So to say that there aren't heterosexual women in Rimworld is something of a "invalid" statement -- there's basically no sexuality in Rimworld, but there are sexual relationships.
Note: This is entirely based off the few snippets of decompiled code we saw, and I'm making the assumption that the game makes a random dice roll when doing it's "do I fancy this person?" check. If it doesn't, then you're right that all women are bisexual, and will happily tip the velvet if an attractive enough person comes along.
edit: To explain this more: Imagine I made a game. It has people in it. For whatever reason I do not track each person's heigh. I simply don't care about it. Then one day I make a bridge, and want to deny some people the ability to walk under it for being "too tall". I could modify the game to give people a heigh and check it when they attempt to walk under the bridge, or I could say to myself "I want 10% of people to be rejected as being to tall, so reject 10% of them". What are the consequences of this? It means that if we see someone fail the "can go under a 6 foot bridge test?" we, as humans, would perceive the little computer people as being "at least 6 foot", but they're not. Infact they could very well pass under the next bridge which is 5 foot 4... Sure, it's contradictory and inconsistent, but it doesn't matter, as it has the overall statistical model that I want.
And it's the same with Rim World here, but because it's dealing with sexuality we perceive it differently. We're brand anyone who has a single bisexual relationship in their lifetime as "always bisexual". (Q: Until they had that first bisexual relationship, were they really even bisexual? This applies for both Rimworld and the real world) and you're saying that because, statistically, the developer wanted 15% of women to be bisexual, they're ALL bisexual. It simply isn't true. Only those that exhibit bisexual behaviour in the game can be considered "bisexual" by the player.
(That doesn't ignore the fact that the developer wants 15% of women to be bisexual and 0% of men to be bisexual, they're different things)