I think a lot of people who make mpreg jokes do it because they consider pregnancy something that 'tames' a person and 'knocks them down a peg'. They think pregnancy is something that marks a person as inherently submissive. Getting pregnant means being submitted to them.
They also think that male pregnancy is a feminist concept not because it recognises trans and intersex men as men, but because something 'feminine' and 'submissive' is forced upon a man. Because that's what pregnancy is to them - something that is done to a person. They think pregnancy is emasculating. Remember this meme?
To people like this it's a feminist line because a woman dominated a man by impregnating him. This is an incredibly fucked up and misogynistic view of pregnancy, and it also reveals the shallow understanding of feminism a lot of people have: masculinity is dominance and femininity is submission, so the idea of a man in a feminine position is inherently feminist, because he is submitted. Especially if he was put in this position by a woman, because then the switch is complete - the woman is the dominant one by forcing him into something.
And yes, I said the idea of a man in a feminine position is inherently feminist to them, NOT the man himself. The feminine position he is in makes him an object to be viewed and discussed, not a subject with thoughts and beliefs. The pregnant man is not a feminist, he's a feminist concept. That, I think, is the core of the problem with mpreg jokes - to people who make them, a pregnant man is an idea, a symbol, a fictional situation, instead of a real human being. They ignore the existance of actual men that can get pregnant in favour of the idea of them, because the first one has agency, and the second one has no agency (because he exists in their head). To people like that, pregnant men = blorbos jokingly impregnated by defying biology, or blorbos in omegaverse AUs, not the living, breathing trans and intersex men. They imagine pregnant men for many reasons: as a sexual fantasy, to make a joke, to declaw and tame them, but never to actually consider what it's like for real life men with uteruses. Pregnancy is just another situation they can put their blorbos in, not a real thing some men go through. So there's no space in their minds for the discrimination we face in gynecological and prenatal care, or the fear of being forcibly impregnated as a way to detransition us. Pregnant men are theoretical to them, just like catboys, so they get annoyed when we remind them that we exist and shoo us away claiming they're not talking about us.
