John and Sarah are reaching the end of their betrothment. Like most modern couples, there has been no shortage of sexual activities. As do many men John often thinks he is "in control". Sarah doesn't contradict this, it's a safe enough fantasy.  
One example of her control is that from the very beginning there has been the mandate that John must always wear a condom for Penis In Vagina (PIV) intercourse. While he doesn't like it, John justifies his compliance by thinking that while premarital sex is fantastic, premarital children would not be.  
Finally the Big Day. A beautiful ceremony. A drama-free reception. By late afternoon they arrive in one the fanciest Bridal Suites in town. John is ready to head to the bedroom and get the marriage consummated. Sarah has another plan.  
Getting John to sit in the living room area, she starts:  
"John, my love, it is time to talk about how our lives will proceed. While perhaps I should have brought this up before, I believe this should be between married folks and not just betrothed.  
"But I am not going to force this on you. If after hearing me out you decide that you aren't willing to proceed, we can go to the courthouse Monday morning and get an annulment. But if you love me as much as you say that shouldn't be necessary.  
"There are some things you will certainly enjoy.  
"First, you will never have to wear a condom again. From this moment forward your Semen is always welcome in my Vagina.  
"Second, provided you don't go overboard, you will be allowed all the PIV intercourse you want.  
"Third, I will offer to give you a child. But this is a limited time offer. And I will only be pregnant once.  
"This is the part you may not like. To ensure that you can only impregnate me once, I will be making a change in you.  
"Starting tonight, and every two weeks until the task is complete, I will be giving your Testicles a thorough Stirring. As I believe you understand, this will cause a progressive decrease in your abilty to produce sperm, until finally there are none.  
"I have arranged for a steady supply of  Testosterone so you will continue to be the strong skillful lover I married.  
"But you understand there is going to be a Race. A Race between your getting me pregnant and my destroying your ability to produce sperm.  
"I'll give you a moment to think while I go get undressed and prepare for your first Stirring. But I hope that your love is true and you will join me for a wonderful procedure and a great life together."  
At first John thought he should be shocked. But then he realized that even though he was fully dressed his Penis was harder than he ever remembered. Maybe not making sperm would be a Good Thing.  
He joined Sarah in the bedroom.