The question is: Why is enjoying sex a value we want to keep, while enjoying punishing is a value we don't want to keep?
I'm not sure who "we" is in that sentence... for example, I suspect a lot of people don't endorse enjoying sex and do endorse enjoying punishment... so I'll speak for myself here.
Value is complex. One of the components to the value equation for me seems to have to do with wanting other people to do what they want to do. Another has to do with wanting sex. A third has to do with wanting rule-violators to be punished.
Call those Va, Vs, and Vp, respectively.
Mutually consensual sex satisfies Va and Vs and is neutral with respect to Vp.
Nonconsensual sex satisfies Vs and antisatisfies Va and Vp. (That is, it causes other people to do things they actively don't want to do, and it inflicts punishment on people who didn't break any rules.)
Punishing criminals satisfies Vp, antisatisfies Va, and is neutral with respect to Vs.
Of course, all of the above are sweeping generalizations and have many many exceptions. Again, I'm talking about myself here. And there are many many many more components to value, and they all enter into this calculation. But adopting a toy world for a second and assuming that these are the only components and they are equally weighted and binary, then it follows that:
Mutually consensual sex = (Va + Vs) > Punishing criminals = (Vp - Va) > Nonconsensual sex = (Vs - Va -Vp)
Further, having values in conflict is uncomfortable.
So I'd expect that I'd find it easier to endorse the first of those (where all the value components are positive or neutral), and that I'd be more willing to edit my value system to resolve the latter two. Or to edit my environment so that the conflict-causing situations don't arise.
Or, as is far more common in the real world, edit my perceptions of my environment so that I'm not aware of those conflicts. For example, if I convince myself that someone I raped did something to deserve it, then nonconsensual sex becomes (Vs + Vp - Va). If I further convince myself that they aren't really people, then it becomes (Vs + Vp). That's much more comfortable.
Or I can convince myself that they actually wanted it, in which case it becomes (Vs + Va), which is also more comfortable.
Etc.
So I think my answer to your question is: some people experience more conflict in their value systems around specific instances of punishment than around specific instances of sex; I expect those people to want to keep the value of sex more than they want to keep the value of punishment. For other people, the reverse is true; I expect them to want the reverse.
Why do those words go together?
Society - and for once, I'm using this term universally - teaches that, if you committed a crime, you should be punished.
But in some societies, we have an insanity defense. If you had a brain condition so that you had no - here it's a little vague - consciousness, or moral sense, or free will, or, well, something - then it would be cruel to punish you for your crime. Instead of going to prison, you should be placed somewhere where you can't hurt anybody, where professional physicians and counselors can study your case and try to reform you so that you can rejoin society.
Wait - so that isn't what prison is for?
No. Prison is to punish people. Is it any wonder that prison doesn't reform people, when we don't want it to reform them? Most people would be upset if prisoners could go in on Friday, and emerge, rehabilitated, on Sunday. When people say, "It would be cruel to punish people who aren't responsible for their actions", they are implicitly saying, "Prison is necessarily cruel; and that's good, because we should be cruel to criminals who are responsible for their actions."
But the more we learn about psychology and neuroscience, the further responsibility recedes into the distance.
Outcome-based justice argues that we should give up playing the blame game, because neuroscience keeps finding more and more proofs that things are "not our fault". Instead, we should write laws that deter crime.
You might think this is what we already try to do. But it isn't! Witness this confused article from the Brookings Institute, Cognitive Neuroscience and the Future of Punishment by O. Carter Snead. Snead objects to outcome-based justice. He summarized all of the arguments for it, yet managed to completely miss their point, concluding where he started from, saying that outcome-based justice is obviously bad because it could lead to being cruel to people who didn't deserve it. (Instead of only being cruel to the people who do deserve it, which is obviously what we want to do.)
Snead understands that outcome-based justice deters crime:
You might expect that Snead goes on to explain why these laws are bad things. But he doesn't! He assumes we can all see that these are obviously bad things.
The Wrath of Kahneman describes a study which asked whether people punish others in order to deter crime, and concluded, No. People are doing something else.
One theory is that people are trying to be fair. Everyone should get the same chances; everyone should get the same punishment for the same crime. John Rawls argues this explicitly in Justice as Fairness: Justice should not be utilitarian, but should instead be fair.
I believe Rawls' view is also the popular view of what "justice" means. And, I will argue in a later post, it is part of a pattern showing a deep divide between two different ways of using the word "ethics".
ADDED: Constant made the point that, while one part of outcome-based justice is preventing future harm from the criminal on the dock, another part is deterring harm by other criminals. This latter part does not benefit from punishing criminals who cannot be deterred. Thus, to optimally punish both criminals who can and cannot be deterred, the law requires a concept of moral culpability, and should punish criminals who can't be deterred more lightly. This is a better origin story for the linking together of morality and free will than the just-so story I'd come up with, so I plan on stealing it for my next post. (SilasBarta may have been trying to make the same point, but I found his comments impenetrable.)
(This post is laying groundwork for two other posts that will go in different directions, neither of which concerns justice.)