I have seen numerous questions about this during the week. Here is a friendly public service announcement for those of you who are just starting on your wedding planning journey.
1. If you extend an invitation to a friend or family member that is involved (in any way) with someone, it is etiquette that you address both parties by name and make room on your guest list for their significant other. Your cousin's boyfriend is not her "plus one," he is an invited attendee of your big day. This rule stands for everyone that considers themselves to be romantically attached: you as the bride or groom does not get to define how serious each person's relationship might be.
2. No matter what people might say, it is alright to limit your guest list. If you don't want to allow your single friends or family to bring a date, because of financial or personal reasons, simply address the invitation to the invited person only. If it is within your budget, it is advised to give all singles a plus one.