How are you figuring this out? It is such a hot topic that many do not agree on.
One school of thought among many: it is disrespectful to ask someone to celebrate your relationship while not acknowledging theirs. But very few people actually consider chronic Tinder daters (the ones who can't commit to one person for more than a week or month) to fit into this category though it is often used as the argument.
While many consider significant others to be a social unit/package deal and NOT a plus one (which is by definition a random stranger), others view them as "if I don't know the other person, I won't acknowledge them or invite them".
Something that many people feel is true across the board is that a truly single person (not in a relationship) cannot enjoy themselves nor will they know others at the wedding to have fun with unless they are allowed to bring a date. Applied to attendants as well. Even though that often proves itself wrong. The random strangers who are invited to eat/drink expensively truly don't care about the couple who is often on a tight budget, so people they truly want in attendance don't always make the list in order to fit these.
Others see the "no ring, no bring" as valid which isolates anyone in a long term committed relationship but no upcoming intentions of making it legal.
What is your game plan on dealing with this?