I put both names on my wedding invitations....it looks sorta rude if you put "Cousin Mary and guest" when they've been together for a zillion years- you know "Cousin Mary's" SO, right? When you see him at family functions, you call him by name, right? Think of it that way...it makes no difference if they are not married. I would only put "and guest" for someone that you are inviting who may not be dating someone, but you would like them to bring someone if they wish so that they are more comfortable at the wedding, and not sitting there alone.
Seriously, how would you feel if your fiance's cousin sent you an invite and wrote "and guest" even though you hang out with the couple every Christmas???? Put yourself in their shoes.
Good Luck!