I think it's utterly reasonable to only invite one person in a couple to a wedding. At the end of the day, if someone is paying for a wedding, they do get carte blanche over the invites. A budget is not endless.
They shouldn't be expected to have to not invite good friend Gerald, simply because they have to make room for friend Bob's girlfriend, who they have never met.
EXCEPTION: members of the bridal party.
I also think that if there aren't children involved, and if the couple live very locally, then it's okay to invite a friend you've known for years to the whole thing, and their partner just to the reception. Especially when aforementioned friend will know plenty of others there.
All this "established relationship" business. That's part of the problem. It can cause a shit ton of arguments and headaches.
At what point do you say a couple are established? When they are married? When they live together? When they've been together for more than a year? 18 months? two years? six months?
For our wedding we basically did exactly that:
married - oh invited
Together and we're friends we know partner well/consider a friend - invited
Together but have only met the partner once or twice - partner only invited to evening (no kids involved)
Travelling a long way to the wedding - partner invited.