We've already put together the guest list for our September wedding and sent out save the dates. One of our friends who is invited (we're not doing generic +1s for anyone due to space/budget) recently started dating (as in, he's officially her boyfriend now) someone that my fiance is kind of friends with (they're all part of the same larger group of friends) and who I've met, but he was not someone we're close enough to that we were planning to invite him. The save the date was sent to the friend only, as she was single at the time (about 3 months ago). Also, to make it an even stickier issue on possible reasons to not invite this person, my fiance told me he thinks they're living together now. Our guest list is already larger than I really want it to be (it's a seating issue--more than X number of people, and we'll have to have tables on the dance floor to fit everyone at the reception), and while in the grand scheme of things inviting one more person isn't a huge difference, if we don't get the 20% no response people say to expect, we'll have too many guests.
My question is really one of etiquette--Do I need to invite this friend's boyfriend? Do I have a leg to stand on not to invite him? I have nothing against him personally, but I feel like every time I turn around someone else is being added to our list and it's gotten too big.