My first daughter was married three years ago.
When the invitations went out, I had two family members wanting to include the partners of their adult children - the adult children (my daughter's cousins) were invited. There was NO plus one included. I dug my heels in and said no. It infuriated me that they would even ask.
Three years on, one of the couples is still together, one is not. My daughter didn't know either of them. One couple lived a distance away, and the other had only just gotten together a month before the invitations went out.
With hindsight, I am glad I stood my ground. Especially since one of the cousins is getting married next year, and I have been able to have a very subtle dig about 'all the hangers on who will want you to invite them as well'
My daughters MIL asked (she didn't ask me, she asked my daughter) if she could invite a friend of hers, so she would have someone she knew there...
Yes, she had her husband and other family members who would be there, but, whatever, I said yes to that, though I wish I hadn't.
My husband and I, apart from $500 'contributed' by MIL and FIL for alcohol, paid for the entire wedding ourselves.
It still annoys me now, because I would never be so rude, and I don't understand how other people can.