... to invite someone to celebrate your public declaration of love, whilst not inviting that person's own parter?
I understand not inviting someone's casual date, or a partner if there is a history of deeply unpleasant behaviour. Nor should this imply that couples should be joined at the hip - it's perfectly reasonable to invite just one person to a birthday party or random celebration.
My gripe is with people who find nothing selfish or hypocritical about inviting close friends/family to honour their partnership, without showing any reciprocal courtesy or respect for their guests' own unions.
I honestly can't see any reasonable explanation for it aside from selfishness - budgets, venues and catering arguments seem to be hollow excuses. Surely the reasonable thing is for the guest list to dictate the venue and elaborateness of the day, rather than the other way around? Hell, even being selfish is fine if you own it and acknowledge that you wanted a certain type of wedding, and guests were a secondary part of that!