I'm with those who don't want to invest in a relationship unless the sex is good, so I usually go for it on the first date. Sometimes it's the only date, often it's not. I am lucky to be highly orgasmic, and if a man can't make me cum first time we're together, then he's probably really shit in bed, and he won't be getting a second chance to prove otherwise anyway. But not all women are comfortable with casual sex (nor are all men,) and nor do all women find it easy to reach orgasm, so what works for one of us won't necessarily work for another any way.
Quite a few of those which have been only one date, it's my choice as much as their's - a couple of weeks back I realised one of them I'd not heard from in three months, and I wasn't bothered, because that was mutual, and I hadn't been angsting about it.
The latest one, we've seen a great deal of each other over a few months, we've met each other's parents and friends. We were in bed within 10 minutes of first meeting. It may yet come to nothing, but it almost definitely would have without the sex.
And yes, maybe only 1 in 100 sex-on-first-date dates end in marriage, but I would think it's similar for no-sex-on-first-date dates - I've had some hilariously awful first dates where there was no chance of me shedding a single layer of clothing.
But it doesn't matter what I think or do. If someone else doesn't enjoy casual sex or needs to feel emotionally close before sex, or is too uncomfortable about the risks of STIs or pregnancy, she needs to do what works for her.