@SageRainWillow I confirm I've seen nothing that can induce pregnancy with woohoo. All the part coding for pregnancy is present only in "try for baby".
I wouldn't be surprised if many people were affected by a common glitch that can easily pass as a game feature.
@MadameLee Yes, it's actually very easy to mod. Woohoo interaction XML tuning files are basically the same as try for baby tuning files, but with pregnancy coding entirely removed. So, if you want to mod these files, you copy and paste the pregnancy chance coding from try for baby into woohoo. You can adjust many variables if you want (you can't create an infinity of them as the game is limited, that's true ; but a lot of possibilities are already present and you can not only change but also add extra variables that don't exist in one specific file but that are still coded in the rest of the game, so you can make any interaction much more complex than it initially was by adding as many extra variables as you want). Your custom file will override the vanilla coding of woohoo. It's a very simple kind of mod to make.
A more advanced variant would be to use Python scripting to inject new interactions such as risky woohoo, which contrary to XML tuning, won't override vanilla coding but will be added side by side (unless if you code use scripts to override files). Mods such as MCCC do this.