Somewhere, Miss Manners points out that if a person isn't currently dating but would like to be, going alone gives them the chance to meet other single people, one of whom they might like better than any of their current acquaintances.
It's generous of the hosts to offer "and guest," and makes sense to accept if you're seeing someone seriously or if you are anxious and want the emotional support, but that sort of "and guest" is an offer, not a demand. Even if you specifically send an invitation "Ms. Tea Drinker and Mr. Coffee Drinker, ourstreet, Big City," we can both accept, both decline, or either one of us accept while the other declines.