The thing about cash is there is absolutely NO ambivalence about how much has been spent. With a gift, you can spend very little and still look like you've thought about the couple (the WHOLE POINT of a gift - you know, it's the thought that counts?) and given to them.
With cash, you're marked out straight away by how much you could afford, and in some cases pushed if you feel like what you could afford wasn't very much. People often contribute more than they can afford, because let's face it, a fiver or a tenner is a really crap, embarrassing gift.
This is why cash is crass and awkward.
I give it when asked. I'd never go empty-handed and if the happy couple want cash, they get cash.
But the whole expectation around gifts, which is inherent in a gift list and the whole cash thing is anathema to me. Again, totally my upbringing! Talking about money, expecting cash, how much one earns, how much your house cost - you just don't talk about it. Asking for money is, I guess, akin to begging for me.
Grim. I don't need to ask for money.
The reason this only comes up on MN as an issue for some people is because they have clearly had different upbringings and mix with similar people. Well, now you realise that lots of other people do things differetly!
And specifying no 'boxed gifts' is really, really not the correct way to do anything.
This is just a ludicrously twee way of asking for money - everyone can see exactly what you're doing.