Honeymoon registries are very tacky, yes. You risk offending a majority of your guestlist by doing this, it's a terrible idea. In the end, you get money, which is pretty much what happens if you don't register at all, without alerting people to your greed.
If you're all set up, that's awesome, no need for a registry. In fact, I would completely forget about gifts and just worry about getting married. When people ask what you want, just politely smile and say you have everything you need.