Four options I know of for male pregnancy.
- Be abducted by aliens. Carrying space rocks in your inventory increases the chances of this. You will have an alien baby, you won't be able to choose the donor, and it won't be a proper pregnancy mesh (your sim will just appear to have put on weight).
- Use Grim's Ghastly Manor from the store. I think you can chose the donor, but as with alien pregnancy it just looks like weight gain.
- Install Nraas Master Controller and use option Sim>>Basic>>Pollinate. You will need to download male pregnancy clothes if you don't want floating heads. There are some at MtS.
- Install the mod we can't talk about and set the options to allow male pregnancy. Again, you'll also need to download cc male pregnancy clothes.
If you don't care about the pregnancy and just want the baby, you can also:
- Engineer a baby in Oasis Landing if you have Into the Future.
- Install Master Controller and use the option Sim>>Intermediate>>Instant Baby. This is my usual option because I keep forgetting to install the cc clothes.