One interesting (side?) effect of
orgasms is that as one approaches orgasm, one's overall
sensitivity decreases, and during orgasm it is almost impossible to feel any type of pain.
disclaimer: I have not conducted scientific studies to test this. All I know is from the personal experience of myself and several others.
It would appear that there is no pain that beats this pain killer. Toothaches, headaches, bruises, you name it... If the person in pain can reach sexual excitement, they can usually be brought to orgasm. At that moment they will feel no pain.
The evolutionary reasons for this seem rather obvious, at least for the male orgasm. When a male is nearing orgasm, which means that his genetic material is probably getting a good chance of continuing for at least one more generation, it would be a crying shame if he stepped on a thorn and had to stop. For the female, the situation is similar, although her orgasm is less important (evolutionarily speaking, of course. In human sexual encounters, the woman is MUCH more important, and should be revered and worshiped and brought to orgasm many times.)
But, there must be a balance. We would not want the couple, once lovemaking has started, to be completely oblivious to pain. So gradually, over the lovemaking, sensitization is decreased, until orgasm, when it is all but complete.
The down side: the second the orgasm is over - the pain returns, full blast. Now THIS is hell. Evolutionarily this makes sense, of course. But forget the post-orgasmic chill, forget fadings. The pain returns, and with a vengeance.
Knowing this, you should be now able to counter that well-known excuse:
Next time your SO says, "Not tonight, darling, I have a headache," you can safely say,
"That's alright. I know just the cure."