In my wedding travels over the past few years I've noticed some inconsistencies in how Brides and Grooms decide to give out the coveted "plus ones." Being single for most of the time following college has placed me in the category of "to plus or not to plus." I find there are a few schools of thought out there...
Technique #1. All friends who live with and/or are engaged to someone are extended an "and guest." This rule is also occasionally stretched to include plus ones to friends whose significant others are known well by the Bride and Groom. All your other friends who may be actually single or in new relationships do not get the added guest--regardless of whether or not The Bride/Groom are actually better friends with the deemed Single guest or the guest living in sin (I'm kidding, but that's how my Gran says it). I find that with rare exception this technique places The Singles in a situation where they are asked to travel to a wedding alone, sit at a table of other forlorn Singles, have no dance partner for slow songs, and stay in a hotel alone.
Occasionally one gets lucky and attends a wedding with other FUN Singles (Yes, Virginia they do exist) and one does not miss having a date in the slightest and may in fact be better off without one. After all, introducing your hired escort to your 50 college friends can be tiring. Most of the time however, attending weddings as a Single is depressing, costly, and takes the spirit of love out of the experience.
I would like to say I understand that per head costs are pricey. Having been a bridesmaid a couple of times I know that your Chicken Cacciatore can cost $120 a person for no good reason. I am not minimizing that at all, instead just explaining the plight your single friends may experience if you do not extend them a plus one. Some final food for thought, female guests are likely giving 3 gifts (shower, bachelorette, and wedding) is it so much to give them the gift of a dance partner?
Technique #2. Friends of a certain age, say over 21, all get plus ones (without attention to their current relationship status). Of course this then runs over into cousins and others over the age of 21 also potentially needing dates. This obviously would multiply quickly, but it may be the most fair strategy I've come across. It's certainly objective.
Technique #3. Friends of a certain closeness with the Bride or Groom (or perhaps their Parents, if they are paying for the wedding) get plus ones regardless of personal relationship status, and friends who are your fringe friends but are nicely being included in the guest list do not get plus ones. Example: All your college girlfriends who were there when you came home from your first date with your Husband-to-be can bring a Gent of their choice. But the girl who you worked with at your latest job is invited as a Single, even though she lives with her bf.
This technique asks you then to sort your friends and single family members into most important, very important, and generally important piles and then dole out those desirable plus ones in tiers. This could help you decide to give only Bridal Party members plus ones, but not other friends. Or help you make a point to your mom that your Cousin Maggie can attend alone, since after all her parents will be there.