Actually, I suggested that the bride and groom don’t ask for stuff, so I see no conflict between my position and the “reducing stuff and waste” argument.
Re the cultural norms thing: when your birthday is coming up, would you say to your friends and family, “just give me cash”?
If you’re in a relationship, when it is Valentine’s Day (which I don’t particularly like but for the purposes of the discussion), would you say to your partner, “no flowers or card please, just give me what you would have spent in cash”?
At Easter, would you say, no chocolate eggs please, just give me the cash?
In the pub, when ii’s your friend’s round, do you say, I don’t want another drink but you can just give me £7 because that’s what a glass of wine costs?
Of course not. Whether you like flowers or chocolate eggs, or consider that you have enough of them already, or you don’t want another drink, is irrelevant. You ask people not to give you the gift or decline the drink offer. You don’t ask for a cash alternative. It’s tacky.