Well Elizabeth, my first response to this is, given their age, and that they are already set up, why on earth would you fund a holiday?
My daughter is being married in a few weeks and she and her partner have said absolutely no to presents.
Their wedding will take place in a very nice golf club, renowned for cuisine, and they have kept the numbers to round 70 I believe.
They have already booked and paid for their honeymoon.
They're going all in on celebrations, but they're funding the whole thing themselves...their choice!
I went to one of my student's weddings years ago. She and her parter were very young at the time, straight out of uni and about to travel.
They asked that guests donate to a wonderful charity. And that's what happened, plus a few presents.
I've been to weddings with wishing wells, ones, where a wedding registry is held at given shops, ones where no directions were given re presents...
My own wedding was a Hootnanny, and it was great fun. I made every scrap of food, including the wedding cake, and we bought the booze.
I think things have gotten out of hand, to be honest.
We shouldn't need excuses to say we can't go.