How would evolution do that?

That's just not the kind of goal natural selection is capable of optimising towards. Natural selection instils the execution of adaptations which on average have historically demonstrated increased reproduction rates. It does nothing else. (Humans, of course, have other goals orthogonal to those of natural selection, and are capable of optimising in favour of them, but natural selection is a blind idiot god that wants precisely one thing and will always aim dumbly at it to the exclusion of all else. We're fortunate that it is so dumb and slow, because it leaves us with plenty of room to do our own optimisation with our brains.)

Can you name any other time natural selection has optimised for the pleasure of a mammal at the explicit expense of reproduction?

You're right, there is no way evolution can do it, especially within single or two generations. It optimizes for adaption and survival, not a pleasure.

Humans haven't been undergoing natural selection for thousands of years, only artificial selection.

The germ line doesn't care about the soma's feelings. If the best way for your genes to propagate was for you to be in horrible agony all the time then that would be the case.

Evolution is not an optimizer in the same sense as a minimization algorithm.

Evolution is simply the differential survival of individuals with different genetic makeup, survival of the fittest. What counts as fittest depends on both the general environment and on the interactions with other members of the same species.

And, of course, without pregnancy there can be no evolution at all.