rebdoniel hit the nail exactly on the head and straight to the point, just as s/he does in just about all of his/her postings to this coffee room.
It is clear as daylight, that his entire etiquette of not inviting a married or engaged person to a social function with his/her souse/fiance[e] stems entirely from the culture of mixed seating, where it is entirely unheard of, not to seat a married/engaged person next to his/her spouse/fiance[e]. In fact it can be considered so embarrassing for a married man to attend a function without his wife, that there are Halachic rulings allowing an “Isho Aveiles” in mourning to attend a Simcha as part of her marriage obligations to her husband.
Clearly this social etiquette’s reasoning is absolutely obsolete in separate seating functions, and more often than not, the spouse who doesn’t know the other party is so bored and feels so out of place in such scenarios.