I call it the “Pink Elephant Problem”: if someone says “Don’t think of a pink elephant”; what’s the first thing you think of? A pink elephant. Same thing applies to tzeni’ut: the more that I say “This is prohibited because of tzeni’ut and this is prohibited because of tzeni’ut”, the more you take innocuous interactions and sexualize them.
“Men and women can’t socialize at a kiddush because of tzeni’ut” – implication: men and women having kiddush together is somehow sexual.
“Men and women need separate entrances to get into synagogue” – implication: same entrance, there’s something sexual there.
All of that is problematic. … You’re sort of putting it in people’s heads that it’s always sexual. … You’re implying that men – and women, too – we just can’t control ourselves, we have no self-control…. We can’t but help get sexually aroused by listening to women. That’s what you’re implying.
When you take the expansive view of erva onto everything, effectively, you’re putting more ideas into their head they may not have even had before, just by pure implication.

Rabbi Josh Yuter, “Current Jewish Questions 26: Music in Judaism“, YUTopia Podcast #106 (23 May 2013) {http://joshyuter.com/podcasts/current-jewish-questions/current-jewish-questions-26-music-in-judaism.mp3}