I must say I don't see this great qualitative difference between TW1 and TW2-3 in handling sex that
@Ajidica mentions. Main romantic relationships/questlines were well integrated into the story in all three. All three also had their share of casual sex encounters with little storyline significance (although there were more of those in TW1). All three featured prostitution, implemented in an entirely similar way.
The only real difference I can see is that "trading cards" were turned into cutscenes, which I think was a decision more connected to larger budget than a change in design philosophy - although I did mention that the "collectible" nature of these cards was indeed somewhat tasteless, so this was indeed an improvement.
The "shallow" nature of these casual encounters, I think, is simply a result of general limitations/challenges of creating and populating an RPG world - they are functional equivalent of a lazily written fetch quest. Every open-world RPG I can think of has some of such. Good ones also have something more. In my opinion, all TW games were good. I don't think it is fair to focus on shallow parts while ignoring that all of TW games also feature female characters who are significant to the world in general and/or to Geralt personally.