Absolutely diabolical case of "emotional monogamy / sexual polyamory" nonsense; Mayumi has the perfect life with a loving wealthy husband and three children, all respectable students, with her oldest being the ace of their soccer team, but the thing is Mayumi's husband is severely lacking in his sexual performance and has a micro dick.
Mayumi had sex friend from high school, the tall and muscular Togo who was a professional playboy back then, got every woman he fancied, Mayumi and her friends were all on his list, but as they grew up and got married the girls all left Togo behind, but not Mayumi she kept him for herself because sex with her husband just wasn't cutting it, and they had plenty of trouble trying to conceive, but how do they have three kids then? Togo is the father of all three (ouch...).
Mayumi is only a tiny bit torn over having cheated on her husband for petty much their entire lives, she does love him, but Togo is indispensable to her. One day Togo tries to make nice with Mayumi, buying her some expensive chocolate and so they have a go at it, but Togo mistakenly calls Mayumi by another name, Misa, which makes her a little mad, giving her the reality check she still is just one of the many women Togo is eating up, but his mojo still makes Mayumi lose herself in immeasurable pleasure, impregnating her 100%, she is ashamed of loving every second of this impregnation with a man that doesn't even bother to remember her name.
In the end Mayumi's husband comes back from a long time out of home on business, she surprises him a with a naughty lingerie, wanting to have sex with him, he is surprised to see Mayumi so naughty but he is eager to do it, she gets lewd and asks him to have another baby, he is ok with it... then Mayumi gets devlish and says making a baby will be pretty hard, he understands but he will try.
In her thoughts Mayumi ends with saying to her husband "Thank you for your (wasted) effort" because she knows his sperm won't be the one to reach her egg (that's what the book's title is referencing).
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Well, at least this poor cuck doesn't push his wife away when she gets naughty and wants some kinky sex, that's the guy I will feel sorry for.