YABU to include anything referencing a gift of any kind in the wedding invitation. That is grasping whether or not you intend to be.
I hate the idea of vouchers or money being requested full stop though. It is tacky and like charging an admission fee. People feel obligated by the occasion to give you money, which makes it distasteful.
However, if you get asked what you would like, and obviously you already know that will happen, why not say something like the following?:
"Oh, thank you very much for asking what we would like, but as we already live together, we have everything we would need. If you would like to do something nice for us, it would be lovely if you could make a donation to [insert favourite charity of B & G] in our names."
If you have set up home already, then that's a lovely way for people to give a gift that doesn't make you seem money grabbing.
You can bet your life that if you go ahead and ask for money (even if it is towards your honeymoon) that a decent amount of the guests will be making catsbumfaces about you. Sorry, but I've been to a number of weddings like this, and people don't give because they want to, they do it because they feel like they have to and it's really very tacky and does get talked about amongst the guests. Charity donations are what I would do, and I'm certainly not made of money. You just should not be asking your guests to pay for a holiday for you.
This will polarise opinion on MN though. The issue has been done to death. There is even someone who has posted here in the past who ask the guests to pay for their own food at the wedding, so you will get all manner of responses.