It is a valid point that this couple might genuinely not want presents at all.
However, instead of just giving them something, ring whichever of the pair you are closest to, and explain that you feel that such an important occasion should be markd in some way. You can then ask if there is some little thing they would like, a dvd, a book, or something one or both have always wanted, but never allowed themselves to buy or wish for.
I would never denote to charity in this sort of situation, unless I knew that the couple concerned regularly did denote to a particular charity. I would not be pleased if someone gave to a charity I would never support myself on my behalf.
If they still say they don't want anything - make a firm invitation to lunch or dinner at your house after they come home from their honeymoon, or you from your holiday.