How problematic is accepting a man as a woman or a woman as a man if they pass well enough? And what problems does that bring?

There's a problem here of not being able to distinguish between the individual and the social. Do I care particularly, that Timmy transitions to Timessa and looks so much like a woman that I end up using female pronouns? Maybe I can't even tell. No, I don't care. Even if I suspect that Timessa is really a man, 99% it's not going to be worth my time to offend/start a fight with a stranger. If I CAN tell, I won't use their pronouns, though. I'll either call them a man behind their back(if it is socially safe to do so), or refer to them without ever using pronouns. But what if Timessa is my boss, and demands, on condition of employment/promotion/raises, that I must include his pronouns, or my pronouns, in an email? Even if that email isn't directed at them? The issue there is that my beliefs and my understanding - accurate understanding - of science are not being respected. I am being asked to lie, to denounce what I know of the natural world, in order to maintain a financial position. That is offensive and a violation of my rights as a human, and Timessa does not have the right to do that, no more than a boss has the right to demand employees refer to him as My Lord and Savior. What if a law is passed demanding I accept Timessa as female, even if we aren't friends and I want to choose not to associate with him(as I would)? The government is thus favoring Timessa's demands over other people's human rights. That's "problematic" to say the least.

Is it just a matter of opinion that people can not be born in the wrong sex or body?

No. If this were possible rather than being a ridiculous, illogical belief held by lunatics, there would be a way to test for it scientifically. I'm all for scientific/medical investigation into the claims of the trans cult. They're the ones who protest such studies. What do you think that indicates?

Is it hateful, rude and disrespectful to call a man who identifies and passes as a woman a man, or a woman who identifies and passes as a man a woman?

No. It is honest. It can be done in a way that IS rude, but devoid of any context: It is not rude to state one's own observations about reality.

Did accepting a man as a woman or a woman as a man have any effect on what we see trans right activists do now? And what were these effects?

Accepting illogical beliefs as facts in the pursuit of "niceness" has had the effect of pushing this argument further into nonsensical demands. Once it was claimed that sex and gender were different, and we could acknowledge biological sex. Then it was that sex isn't clear cut and exceptions exist. Now many of them claim sex isn't real at all, there's only gender. Ideologues and bullies DO NOT STOP because you give in to their demands. Terrorists cannot be appeased. They will keep demanding for every victory, swearing that they'll stop threatening your jobs, your social connections, your life if you just give in, repeat the cult mantra this time. Just this time. It's rude and hateful not to, and we will destroy you if you don't. We're not asking much. Come on, give in. Just give in, and it'll be all over.

It will never be over. They will destroy all of society, kill every woman on earth before they admit that they were wrong about anything. The only way it ends is for reasonable people to say NO. We will not concede to your unreasonable demands. Not on this one, not on that one, not on ANY of it. NO. NO. HELL NO.