My parents were married 61 years ago. They were young and poor, and had about $10 to their names on the day they got married. They planned a honeymoon to a place about 3 hours away, made a reservation and all, but had no money to pay for it. My father assumed people would give them cash at the wedding, which they did. It was not in lieu of a gift, but just a tradition that (mostly) men would give the groom $5 or $10 or if very generous, $20 in cash. That funded their honeymoon, but of course they were going to an inexpensive hotel at a resort area for 2-3 days, not a trip to Hawaii or Paris or Jamaica. Now getting $5 or $10 at the wedding is not going to help the couple plan for a big bucks trip, but I don’t think it is appropriate to have a Go Fund Me. A honeymoon registry where people buy a dinner or an experience is better, if you ask me.
My friends had several options: a traditional registry, an option to contribute to trees they were planning on planting in their yard, or 3 charities that they support that you could make a donation in their honor. I thought these were pretty good options as they had a rather small house, they were older so each had their own crap, and they each had elderly parents and would inherit even more crap when the parents died. There really was quite an assortment of thing as many of their guests where from around the world so rejected these options and did their own things, some were artists and did their own things, and some just did what they wanted anyway.
Maybe 40 years ago a cousin got married and long before it was the ‘in’ thing to do, she requested cash so that they could buy a mobile home. I was horrified, not only that she requested ‘cash’ but that she was going to live in a mobile home! At the time I was getting ready to go to college and thought she was ruining her life by getting married and living in a mobile home when she could have gone to college!