I think you're right. Most dinner parties don't come with a general "bring a friend" to add up to an even number of people, but someone might well ask "can I bring my new girlfriend?" or "this is for me and Mary, right?"

From that angle, saying the party was "for my friends and their plus ones" sounds like a deliberate attempt not to identify Mary and Amy as a couple, whether that's because the LW is in denial or because, as the_rck suggested, there are circles where class bias is more acceptable than homophobia. If the LW has been reading Dear Prudence for long, she might have known that "I don't want her bringing a girlfriend" wouldn't go over well, but thought "she's too poor for us" would.