I don't understand how you are unable to understand that your perception that trees are all either male or female is incorrect
Seanie never made that claim
or what the sex of trees has to do with human beings.
The only thing it has to do with humans is
all anisogamous species have two and only two sexes; humans are anisogamous, and so are many (not all) trees. So are the vast majority of vertebrates, by the way. Anisogamy evolved over a billion years ago, it's a highly successful evolutionary mechanism.
Some animal species actually do change sex.
So what? Those species that change sex are still male or female, there is no third sex involved. Additionally, humans are gonochoric and cannot change sex.
Male seahorses gestate and give birth to offspring.
So what? Male seahorses have the reproductive anatomy that produces small gametes - that's what makes them male. Male emperor penguins sit on eggs for months at a go, they're still male.
There are lots of exceptions to your dichotomy.
None of what you mentioned is an exception to the dichotomy of male and female being the only two sexes in anisogamous species.
You brought up trees. I have tried to explain to you that trees are not just male or just female. Some have both male and female reproductive parts. I agree that has nothing to do with humans.
Anisogamous species that are hermaphroditic (regardless of whether they're sequential or simultaneous) still have males and females. That the reproductive anatomy exists within the same individual in some species doesn't negate or alter the observation that male reproductive apparatus and female reproductive apparatus evolved in them, and that these two sets of organs are distinctly different.
Do you know how we can tell they're hermaphroditic trees? Because we know and understand what the male reproductive parts of a tree are, and we know what the female reproductive parts of a tree are, and we understand that the male parts are different from the female parts.