In conventional dating, the boundaries and expectations are not very well defined and the boundaries change.
One unwritten rule in conventional dating is that you get to know each other before you fuck. And if you are a straight non hormonally challenged guy - getting that pussy is always a consideration. Making sure that the pussy doesn't get mad and lock up is always a reason that guys will put up with all sorts of shit - hence the term "pussy-whipped". As an example, no guy with a man card ever actually wants to go XMAS shopping at a mall. EVER. And, of course, there are expectations with respect to monogamy if you get serious... and an expectation of getting serious if you put in enough time. In other words, boundaries change.
In P4P, the boundaries are pretty well set. You may have folks who push them BUT everybody knows what the boundaries are supposed to be and they don't GENERALLY change. Ok - that's a generalization but I think it has truth in it.
Since, in P4P, all we know about each other is how we behave BCD - then sex is a natural and reasonable conversation topic especially between rounds. It's what we have in common. It's also a way of figuring out sexual boundaries in a non confrontational manner. I have never asked about the worst session - but I have asked about the best/wildest session...
Another "unwritten rule" in conventional dating is that a guy being direct about what you want is not considered gentile - at least until you get to the "monkey sex" stage. In P4P, directness works well - or at least it does for me. Obviously, you can't be a dick and expect good results but at the same time, no provider I have run into wants to play twenty questions either