I am friends with a married couple - Mr. and Mrs. Smith.
I like Mr. Smith a lot. We've been friends for as long as I can remember, we always hung out together, drank together, played cards together. But when Mrs. Smith came along, I found I didn't much like her.
Now, I would like to spend time with Mr. Smith, but I don't want to spend time with Mrs. Smith.
She annoys me.
I have three choices on how to deal with the situation so I don't have to interact with Mrs. Smith yet can have as much fun with Mr. Smith as I want.
I can wait until Mrs. Smith leaves the house, then go over and visit with Mr. until such time as she returns, when I will bid a fond farewell until next time.
I can go over to the Smith house to visit, but slip a mickey into Mrs. Smith's drink so that she falls asleep shortly after I arrive. That way, Mr. Smith and I can have a marvelous time until she wakes up, when again, I will bid adieu.
I can kill Mrs. Smith. Then I don't have to deal with her ever.
From a moral perspective, does it matter which I choose?
If Mrs. Smith was your wife, would you have a preference in how I handled her?
Mr. and Mrs. Smith are the twin gifts of pleasure and fecundity, respectively.
Choice 1: NFP. I just wait until fecundity leaves.
Choice 2: Temporary chemical and physical barriers. I drug or bind up fecundity so it can't interfere with me.
Choice 3: Permanent sterilization.
Now do you see the difference?
BTW, I stole this story from a friend of mine who thought it up a few years back when he was teaching NFP.