While I certainly don't believe that strapless = "tacky" (I despise that word) or "disrespectful", I do think the posters who feel that way have emphasized that if you get married in the church they consider it disrespectful. I don't see them inferring that church weddings are the only way to go and I know some have even specified that strapless doesn't bother them in a non-church ceremony. I think if church weddings are being emphasized it's because that's where they feel it becomes disrespectful.
The idea that they are boring would apply to all ceremonies. Yay? But hey, why are brides getting all the scrutiny here? If we're talking boring, I haven't seen much in cutting edge wedding attire couture for the grooms.
Okay, so these posters aren't making weird blanket rules for all weddings ever, they are just making weird blanket rules for all religious weddings. Still not cool.
This attitude treats 'getting married in a church' as if 'church' only means one thing - as if religion itself is a monolithic entity where the rules, standards, and expectations are always the same. This is obviously patently false. There are not only many different religions, but within each religion many different denominations or sects, and then within those different individual church communities all with potentially *wildly divergent* standards of what decorum is expected.
It's not appropriate or polite to make a blanket statement that strapless gowns, or jeans, or coffee, or ipads, or uncovered hair, or pants on women, or men's shaved faces, or tattoos, or makeup, or anything else is "disrespectful in church" because (general) you don't get to tell me, or anyone else, how our churches-that-you-do-not-attend are supposed to function. You can say "this would be considered disrespectful in MY church". You can say "I would not be comfortable in a church where this was accepted". But you don't get to say "this thing is disrespectful in church" as if you are speaking for the monolithic entity that is 'all church'.