why do trans people get so upset when women assert their rights to sex based spaces?
They don't get upset about this at all. What they get upset about is the exclusion of trans people from these spaces.
And that's the same thing, and the root of your dishonesty. Since you and everyone else on the planet knows this is absurd and dishonest, you are losing.
Women have a right to single sex spaces. Men are excluded because they are not women. If women let men in, they are not asserting their right to single sex spaces. You think male entry to female spaces is a human right. It isn't. You think women should have no right to exclude men. They should.
Trans identifying men can use male spaces or unisex spaces. The latter is all there is when anyone can identify their way into either one. They are not denied anything except the capacity to force their beliefs on unconsenting women and prevent them from having their own suitable space.
You know all this, of course. I don't know why you think male self image trumps all, but you're losing more every day. You cannot win on lies and absurdity and you should be grateful you got it for as long as you did.