Plus ones!

Your wedding, your choice.

I think plus ones can be kind in certain circumstances - e.g. a guest who won't know anyone else at the wedding or one who has been recently widowed/divorced/separated and who may find it extra-painful going to a wedding alone. But it's still not necessary, and it certainly shouldn't be at the expense of excluding people you actually want. We had a couple of recently divorced/separated people at our wedding who were fine with coming on their own. And in my single days, I went to several weddings where I knew no one other than the bride or groom, and I managed just fine.

We specified to our guests that we were only inviting plus ones who were married or in long term relationships. However, it's your wedding so it's entirely up to you. Whatever you feel comfortable with.