I think this because this is what Veronica has repeatedly demonstrated throughout the game, as opposed to acting like a person with equal preference towards both sexes, which is what people normally picture when they say someone is bi. It seems to me that people are overblowing this notion that "she is bi" when in reality, if you assigned a scale to the level of interest in physical features of guys and girls, one would be around 1 and the other around 5. The fact it's supposedly just feminine guys that can get her interested, and even then it's a rare occurrence, it's telling.
The thing is, if your interest in a partner is ever, at best, lukewarm, because most of their features just don't turn you on, then the prospects of a long-term relationship that doesn't end when a person shows up who ticks all your boxes as opposed to just a couple of them, is minimal.
Sure. But if then given a choice between Mina and otherwise identical redhead-Mina, do you think she'd prefer the former, or the latter? Which would she find more appealing?
Interest isn't something that has identical strength for all possible subjects. It's proportional to how well the subject meets all our preferences.