yid.period-
First of all, I commend your sesitivity and caring for yenem’s feelings, and hope this mida continues with your (IY”H, bs”t) future wife.
It’s not pleasant getting a “no”, but it’s part of the shidduch scene – just be as kind about it as possible.
It’s quite possible the girl doesn’t even know you’ll turn her down – usually the guy is asked if he’s interested first.
I understood your post was tznius related, and that it was something that you regard as quite black-and-white wrong.
Even in situations like this, simply saying “She sounds like a nice girl, but xyz is very important for me, so I’ll have to say no” should’nt be offensive.
There are many tznius-related factors – I won’t discuss them here – that different people have different levels of observance/hakpoda on.
Hatzlocha raba.