When young couples are not registered, i think most people figure out a check is the way to go. I don’t think there’s a need to set up a fund through a third party website.
There’s always the indirect, slightly disingenuous approach, which has been used quite effectively by my young relatives who were recently married.
When I asked the bride’s mom where they were registered, she said, “Frankly, I think they would prefer cash because they are not yet settled, but they don’t know how to communicate that without sounding crass. Any suggestions?” I then happily brainstormed about how they could get that message out.
I am now starting to think that that WAS the brilliant solution for getting the message out – when friends and relatives ask about gift registries, ask for advice about how to get the cash-preferred advice out to others!