Forgive me, but after reading the title "pregnant by woohoo?" all I could think of was, well, yeah, that's pretty standard.
Joking aside, I have no idea if it happens or not. I know there was chatter about it in another iteration of the game and some painstakingly explained that the pie menu might be off slightly, so that while you thought you were selecting "woohoo", "try for baby" was activated. Or something along those lines. In my own experience, I never ran into this problem. Sometimes, you can have your Sims "try for baby" and it fails, but I have never had a Sim get pregnant after just "woohoo". I view that as a more responsible form of woohoo, as in using 'protection'.
Perhaps, others have more direct knowledge and can better answer your question. Hopefully, without a fit of giggles. :open_mouth: (Hey, be nice to me, I've got the flu!)