So whats your problem? Either give them what they want or don't give them anything. If they are offended after you making the effort to go to their big day, thats their problem, not yours.
Not everyone feels capable of taking this 'hard line'. Whether you agree with their feelings or not, the fact is that requesting cash instead of gifts embarasses some people into spending more than they can afford or forces people to turn down an invite because they can't afford a cash present . Personally, I would hate to feel I'd put my guests in this position, it obviously wouldn't bother you. We'll have to agree to differ on this one.