Is it true that some humans are r-selected, which means they’re promiscuous and want to have lots of children, and some humans are k-selected, which means they invest in a few children?This is not what I'm saying. If you look at any human's ancestry, you're going to find both influences. The last thing I intend to say is that there are meaningful differences between humans (either at an individual or group level) on this front. My point is that both sex drives exist, because both are a part of our evolutionary story.
Humans as a species are absurdly k-selected. How many species can you think of that have young that are totally helpless for their first decade of life? r/k selection theory would argue that we wouldn’t have casual sex, because humans are not remotely r-selected on any conceivable level!
My argument is that all humans are both, each to some degree, although the K-drive is more socially acceptable, civilized, and progressive... and probably the stronger one in most of us.
Being r-selective is actually very adaptive during times of ecological stress, such as after a population crash. K-selection is more fruitful when population is at saturation and the only available growth is civilization's progress. I don't intend to moralize on this. We needed both drives to get us where we are. You're right that, as applies to any specific animal, the r/K selection theory is a bit simplistic because almost all advanced organisms exhibit traits derived from both evolutionary imperatives (quality and quantity). For example, trees are K-selective in terms of being durable and large, but r-selective in terms of having many seeds.
As for "we wouldn't have casual sex", I'm not so sure that there's an either/or. Humans are an advanced animal and, given both our cognitive capability (and flexibility) and our numbers, it's quite reasonable to assume that human behavior includes the full spectrum of strategies. It's not unreasonable to suppose that humans are predisposed to r-selective behavior when young (15-25) and K-driven behavior when older (25+).