That's missing the entire point. The whole point of their religion is their belief in God. A low cut dress might be an affront to their God, but nowhere near as much as making a vow before a God you don't believe in!
Oh - I agree! The problem is, no-one can enter someone's mind and say "Here - you're lying about believing in God!', even though you may know they don't believe"
However, you can look at a dress and think "What sort of slack-fannied trollope wears that on her wedding day IN A CHURCH?!"
It IS about respect for the beliefs of those people who have a faith.
Unfortunately CofE churches do not get the option of refusing to marry a couple if one of them is on the electoral role of the parish - and for the person who posted previously saying the churches were hypocritical because they were happy to take the money - it costs a HELL OF LOT to heat a church. Bell-ringers have to be paid (Would you do it for nothing? No? Well, neither will they.), flowers ladies don't usually charge for their time, but if the bride decides not to have her own flowers, most churches still will make the effort to have appropriate (ie white) flowers available for a wedding and the flowers aren't cheap. Vergers and organists have to be paid (organists are a dying breed, and aren't cheap) Similarly if there is a choir, they get a small remuneration.
It amazes me that people who are prepared to pay £000's for a dress and more 000's for the reception and even more £000's for the honeymoon, get very arsey over a couple of hundred quid for the use of a church. they seem to think it should be free.
Anyway - rant over.
Wear what the hell you like to a non-church wedding - look as slutty as you want in the name of being "sexy"; but if it's a wedding in a religious house of NAY faith/ denomination, please respect the fact that you are in a sacred space - even if you don't give a monkey's about it yourself, other people do. Show some respect