Why? Cramps, orgasms, & even labor contractions are all basically the same muscle groups at work. Cramps are most like labor pains, both serve a similar purpose. Somewhere I saw that the contractions for a woman's orgasm push the cervix into the pool of semen in the vagina, helping the sperm on their way. How, you ask, would a man know all this? Wife has suffered from irregular periods all her life, & we found that when she was late a vigorous round of sex would bring on "George" (as she calls it), usually by morning. The troubles with her cycle seemed to extend to monthly bleeding even while pregnant; all this & more taught me how to help her out with massages in the right places, when she hurts it's no fun for me either!
As to sex during her period, she's too fussy about making a mess to let me touch her, I've hinted at trying and got shot down quick!

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