How to politely ask for money instead of gifts?

If you don't believe in materialistic things, then you can easily exclaim your opinions and feelings. I am sure they will understand.

Of all the weddings that I have attended I see few gifts and a lot many envelopes. So maybe you won't have to pose it to them, the dough shall come on it's own

By the grace of the almighty (i am guessing) you have the comfort of all materialistic goods. Ask them to make a donation to your favourite charity.