Sims can woohoo autonomously if the option is available (high enough romantic relationship) so I don't see why they couldn't as roommates unless they have no access to woohoo spots (say locked out of each others rooms, no closet or other location).
Pregnancy is a different matter. Sim's can't autonomously Try for Baby in a vanilla game. If you have any kind of risky woohoo mod, or allow autonomous Try for Baby, then it could happen.
I suppose NPC roommates could get abducted by aliens too, and come back pregnant that way.
