I think it's a fairly reasonable request. If you make sure guests are told well in advance and reminded at some point, then chances are they will stick to it.
If you have ushers, advise them to courteously remind the first few people they see reaching for their phone and word will get around.
It should become self enforcing after than - if a guest then goes on to post a photo on social media then they should realise you will get to find out about it.
If you aren't confident in that approache, you could ensure that your wedding photographer is using a completely manual film SLR and then detonate an EMP.
Or, more realistically but possibly still not terribly legal, but a signal jammer would prevent people from uploading photos during the course of the day itself. ( -not that I'm condoning it)
I think it is a shame when a wedding takes places and hundreds if not thousands of bad photographs are unleashed onto the internet immediately afterwards.