Because the majority of the population is not bi-sexual. Generally speaking, most people have a single sexual preference. So when you have a story showing two separate characters having a sexual preference, I think most people are going to assume that that is their sexual preference is.
It is totally possible for someone to be bi. Or for someone to not know their sexual preference until later on. That is 100% true. But again in story, we can only go off what we are shown. So for many, seeing romance plots shown early on for them, kind of made it seem like that was their preference.
And because Bryke couldn't have a plot that showed them changing preference or growing their relationship into romance, it might come off as forced for some.
Whereas if you don't show any romance, by default we don't know what a characters preference is. But technically speaking you are 100% right. You can't assume that someone is not Bi, just because they had a straight romance in a story. It's fiction. A character can really be anything.
I'm just saying, I understand why people would think it feels forced.
Also let's be clear, I don't think this just applies for women dating a man. I see it the same as a man dating a women, and then having a relationship with a man. This has nothing to do with females (at least for me). At least for me, I'm purely saying this from a story perspective, and how we perceive sexual preference.