I mean, both are totally unrealistic. Yes, yes, I understand if you have "magic" in your world, it's soooo easy to make it so a guy can become pregant. (Although, to be honest, that happens so often that it's rather lost it's punch. It's become like the company known as V. C. Andrews writing incest.) But, if it's sooo easy to suddenly make a male capable of getting pregnant and having a baby, shouldn't it be just as easy to make him female?
You know... I suggested that once to a Mpreg writer who told me, "That idea is too ridiculous, no ones going to believe a guy can be turned into a woman." Okay, you're right, it is too ridiculous, but is it really any more ridiculous than a man becoming pregnant?
Now, the idea of a same-sex couple being parents is not at all far-fetched and there are a lot of perfectly realistic senarios in which that can happen. Adoption, surrogate parent, for a sci-fi twist, you can always have the child be a clone or a test tube baby... so on and so forth. With any of these things you can still go into slushy, gooey, stories about these two people being parents to this child. Adopted children, once in the family, are just like birth children, any angsty situation that can happen to a birth child can happen to an adopted child too. Of course, with an adopted child, you can always throw in the angsty little, "Birth parent wants child back" if you really need to make your readers sob.
This leads me to conclude that the important part about Mpreg fics are the guy being pregnant. Why? You can' t really expect people to read this and go, "Mmm... okay so guys can get pregnant... better make sure the male pill gets off the ground soon!" No one is going to read an Mpreg story expecting that these two people are going to be their neighbors.
This leaves the shock value. "What can I do to be totally outrageous? I know, I'll make a GUY get PREGANT!"
That worked the first dozen times, but now it's so common place it's a yawn. Yes it is. When something has a cute little slang warning term, like... Mpreg, then it's commonplace. So, your mpreg fic is no longer shocking, it's boring, you've lost that edge.
Oddly, I don't hear of too many fics where a female couple ends up with one/both pregnant and it's proven later that the baby (ies) turn out to be the child of both of these women genetically. I think this is because the idea just seems too... plausible. Women can get pregant, big fat hairy deal. The fun is the BIG SHOCK OF A GUY BEING PREGNANT! Move on. Mpreg will never be shocking again. It's too "been there, done that."
Time to find the next shocking thing for fanfiction. Which, I'm starting to wonder if it's even possible.
Oh, and in other news, I walked six miles today. Not the biggest news in the world, but I'm pleased.