Can I make sure I have this straight?
One) You can make guys look feminine and vice versa.
Two) You can change household sims from one sex to the other once so long as they are within the proper age range, which right now is teen and YA.
Three) You cannot change a sim's pregnancy capabilities, however the change in sex will do so.
Four) If you want same-sex couples to have kids together and be legal within the dynasty, then you have to a) move in the spouse at the appropriate age, b) change the sex of the spouse so that they have the opposite equipment, so to speak, and c) change the sim's presentation to the original sex to try to retain the sim's original look and thus the same-sex status. I don't know, it seems way more complicated and unnecessary than simply changing how a sim reproduces.
I would like to point out that woohoo, three trimesters, and pop the kid out is exactly how the game has guys do it already via abduction. I mean, yeah, alien abductions get different moodlets but the mechanics are basically the same. The only thing that's changed is that now they can be impregnated by anyone with the 'impregnate sims' button checked if they themselves have the 'become pregnant' button on.