Again, I really don't even see this as an issue. Does your SO need to go to every party you're invited to? If so, that sounds like a very dependent relationship. Remember, the lynchpin here is that it's a logistical issue, nothing more: it's not that she doesn't want to invite the SOs per se, it's that she doesn't have the room. That's the major point. More so than it's an issue of manners, it's an issue of physical accommodation. Any SO that would question this scenario and take issue with it, I would question their commitment to the relationship. People need to have their own lives and any SO that feels slighted by this request seems to me to be have other trust issues that they need to work out on their own.