I wouldn't put it quite like that. For me it's a combination of how close you are and also whether inviting someone without a plus one means they'll be travelling/staying on their own and/or won't know many people.
So for example, I think it's not OK to not invite the partner of your brother or sister, even if it's a new relationship or you're not close to the partner, because your brother or sister deserves more consideration than that.
Younger cousins who aren't in long term relationships and are travelling as part of a family group with their siblings and parents? Fine not to give them a plus one. In this situation I'd give them a plus one if they were in an established relationship with someone I'd met.
Friends who are part of a couple? Invite them both.
Friends who are single? A plus one to bring anyone they like if you can stretch to it, otherwise invite them on their own but think very carefully about who to put them on a table with to make sure they have a good time, and try to introduce them to other wedding guests beforehand if possible, e.g. at hen/stag do.
I once got invited to a wedding without my partner. It was on a Friday, in a very remote part of the country, would have necessitated a two night stay in a hotel, I didn't know any of the bride's other friends and I wasn't invited to the hen do. When I realised my partner wasn't invited I told the bride I couldn't make it and she accused me of only not wanting to come because he wasn't invited. Well, yeah. That tipped the balance from "it's a lot of effort but I'll come and celebrate with my friend" to "meh, it's a lot of effort and I don't fancy it".