I don't know what the hell is causing it, but my two Sims enter a romantic relationship (go steady) and in their status it says that they're boyfriend and girlfriend, but they cannot even sleep in the same bed together, any romantic action that they do, it makes a memory of "decided to go steady with X"

My guesses, without more information to go on, is that either one or both of the sims are teens or they just don't have a friendly relationship high enough yet to share a bed.

For non-blood related teen sims to actually be able to share a bed (for sleeping), the SleepFreedom mod is required, the bed must be an EA bed from the base game or one of the EP/SPs, and the bed must be replaced with a new copy of itself if it existed before the mod was added.

If your sims have a low friendly relationship score, no matter what their romantic coupling status may be, they won't want to share a bed for any reason.