Emma, your Stepsister's Wife
You're about to have sex with a woman you can't stand, and the feeling is mutual. The lady in question is Emma, your stepsister's wife.
She considers all men to be testosterone fuelled chauvinistic brutes responsible for the suppression of women throughout history. She wants to ban all pornography because it exploits women. Same with prostitution. Ban strip clubs. Ban sex shops. You get the picture.
She and Katie want to start a family. Adoption is out of the question. Emma wants to experience pregnancy, and motherhood, and to feel the special bond between mother and child. Katie wants to feel a close bond, too, which is why she wants you, the only man she's ever loved, to be her child's natural father.
You expected a visit to a fertility clinic, but Katie, a vegan, has become a committed naturalist. She's convinced that a child conceived naturally will be stronger, fitter and more resistant to illness than one conceived by IVF.
According to Katie, there's a three hour window in Emma'a ovulation cycle when conception is most likely...and tonight's the night.
Any sexual activity must always be with mutual consent.