I don't drink, and I find the idea of a cash bar tacky. (Yes, I used the horrible judgemental word...)
I find it to be unacceptable for a number of reasons. The most important being that you are hosting an event. You do not invite guests and then charge them. Why not make them pay for dinner or a slice cake while you're at it? That would save a ton of money and the reasoning is the same, really. You are inviting these people to see you get married, yes? Not to pay for them to eat on your dime.
Also, frankly, if you can't afford your wedding, have a smaller event and treat your guests as guests. Invite 75 people instead of 100 and then show the people you care about that you actually care about them. I realise this interferes with many people's grand wedding dreams of being the centre of attention amongst hundreds... Or cut back somewhere other than money on your guests. Get a cheaper gown, take a car and not a limo, get fewer flowers. Of course, that means giving up something of your own rather than something for others, which is a big deal, I know. Or have fewer alcoholic options, but then offer them openly. Or if its vitally important to have the huge guest list and all the trimmings then save up and do it properly.
More and more, couples seem to want other people to pay their way. Parties and showers, Jack and Jill fund raisers (seriously, who fund raises for a wedding?! It's not a charity!), "cute" poems and registry cards shoved in guests faces so they are fully aware of the expected price of admittance when it comes to gifts, cash bars, money dances... When will it end? I sometimes think couples should just sell tickets and be done with it, then everything will at least be above board and couples can cease thinking of clever ways to extort money from their friends and family.
I know I sound harsh here, but when did getting married become less about celebrating your love and more about putting on a show and making a buck? Miss Manners once said that she doesn't care if you serve water at your wedding, just do it graciously.