Bed Assignment's

@TheBitzii 

One thing you haven't mentioned is the friendship and romantic levels between the 2 sims assigned to the bed.  Chances are you are playing two new sims who were married in CAS when you made them or one of the premade households.  In my experience, CAS sims, even those you marry usually end up with low friendship and romance when you place them on a lot right out of CAS.  Most of the premades are the same way in a new game.  That doesn't change until you play them and improve the relationships though socials.  If the sims are not at least good friends, they will not sleep in the same bed no matter what you do.  What will happen is when the 2nd sim goes to the bed and gets in, the first sim will get up or the 2nd sim will give you a route block thought bubble and not get in.  If you send sim 1 back to bed, sim 2 will get up or sim 1 will give you a route block thought bubble.  Who's assigned to which side of the bed doesn't matter nor does marriage for the most part.  The game has always been this way.  To get 2 sims to sleep in the same bed at the same time, you have to get the relationship levels up, with friendship being the most important.  Or, otoh, if you play with mods, there are several mods that will let sims sleep in the same bed without any relationship between them including sims that don't know each other.

Hope this helps.