Sex. Sex sex sex sex sex sex sex. Why is it so difficult to talk about? Whether you want to do it, or have done it, or have done it but don't want to do it again, or have done it and do want to do it again, it's always an awkward subject to broach. Part of it is timing, I suppose: when, except for right afterward, is an appropriate time to say, "That was very enjoyable"? - this is scarcely a remark that can be made naturally or casually at the water cooler, say, or whilst getting a hot dog from a street vendor. And when is it ever an appropriate time to say, "I don't want to do that again with you"? Then, of course, there's the issue of one's own feelings. Even if both parties appear to have enjoyed themselves, one risks a small chip of self in saying, "So, are we going to do this again?" And if one of the parties does not appear to have enjoyed her or himself, how much riskier to say, "Do you no longer wish to do this with me?"
It's the privacy and the intimacy of it, I suppose. We pretend these days that sex is an open topic, and some of us pretend that it's a thing among things. And indeed, I think it can be. But even when it's a thing among things, it seems something instinctual or very nearly so in us recognises that it's private - that is to say, something to be broached at particular times, and in particular places. And something recognises, more clearly, that it's an intimate act, no matter how casually we do it: to have someone tell you you tie your shoes badly causes a momentary twinge, but to have someone tell you that they no longer wish to have sex with you causes much more. Both acts are allegedly casual, but apparently one is less casual than the other.