Very easy to reproduce this bug and it doesn't really warrant a super detailed description: couples with more than one wife/husband still have the "slept alone" debuff regardless of if the bed is in the same room or next to the double bed or anything like that, they literally have to be in the same bed as the wife/husband. Here is a Reddit thread from someone with the same problem: https://www.reddit.com/r/RimWorld/comments/oohws4/polygamy_beds_in_ideology/ (https://www.reddit.com/r/RimWorld/comments/oohws4/polygamy_beds_in_ideology/)
I think this is more of a feature request than anything else. I held off on the polyamory marriage as I was strongly suspicious that what you described would happen. ("Not fully implemented just yet")
We're going to need bigger beds, or other provisions, like you alluded to, for "linked beds" in order to make that work. Option three if so long as the relationship is "one to many" and not "many to many" they just provision for the poly pawn to be assigned to multiple beds and that pawn decides which bed it will hop into by whatever logic the devs decide to give it.
I'd say it's a bug, or at least a not well-thought-out feature, right now. What's the point of allowing multiple wives/husbands if it's going to give a negative moodlet to all of them?
Being married to multiple partners already removes the "barrack" moodlets if the correct precepts are in play. Maybe have it check to see if the beds are right next to each other, and if so, have it remove the "want to sleep with x" moodlets too.
@dhakkel: We are on 1.3.3074 and it (hopefully) fixed that. This report was for 5 fixes ago.
Having the lover anywhere in the same Barracks should be enough now.
Either way, this was behavior as intended originally. If it has been fixed, great!