But isn't the polite thing to do to invite a single person with a guest? Then the single person can decide if they're uncomfortable attending solo. Particularly if the person is coming from out of town - this rule does not so much apply in the case of a single cousin who'll be there with four or five siblings who may or may not be married - that case, I would say, would depend upon your budget.
We always invite single people with a guest to our family weddings - they do not always bring a member of the opposite sex, sometimes they come with just a friend. Also, widows who need a driver will generally bring one of their adult children as their guest. Consideration for the situation of your individual guests is very important.
I guess it has to do with what your guest list looks like to start with. And that's a function of region, economics, taste, culture ...
We don't, in my family, general invite to weddings a lot of people who will be strangers to everyone else, so there's no reason for anyone to feel uncomfortable. However, that's not a moral principle or anything, just the way we do things.
If you don't know people well enough to be comfortable at a wedding reception, why would you want to attend their wedding at all, anyway? Just decline the invitation promptly, and go out to dinner and a movie with someone you like.