PPs have said about men losing interest when women are liberated etc etc etc
I don't think it works like that, but what I do think is this:
Men work differently to women. Sex is a physical thing that can happen quickly, but getting romantically interested in someone takes more time. After sex men feel a bit like 'been there, done that' and pull away - this is a hormonal thing that just happens. It's like before sex they are a balloon, puffed up with anticipation and the sexual draw. Having sex pops the balloon.
The lure of sex is what subconsciously helps keeps them moving forward and motivated to meet up and get to know someone and over time they get more interested in /connected to the person.
If sex happens quickly then they have reached the biological goal without any attachment having built up first. The balloon pops. And then, unless they have had a blinding flash of love at first sight, that tends to be that.
Women, however, have more oxytocin released after sex which makes them more attached to the person they had sex with.
So if a woman wants something to last, it's better to wait. Nothing wrong with having sex in the first couple of dates if that's what you want, just be prepared that it might kill it off.
OP, I'm sorry but just walk away with your pride intact. He is being polite and letting you down gently by taking a few hours to reply. It's possible he might get in touch again, but not very likely, so forget about him until/ unless he does. All the talk about a 3rd date was when his balloon was still inflated with anticipation.