Doing a friend a favour

[fatherted]

Last year my mate Dave found out that he couldn't have kids.
He was absolutely devastated, so being the kind of guy I am, I offered to have sex with his wife, which they both agreed to.
Dave called me today and said, "Look, you've been shagging my wife twice a day for the last six months and she still isn't pregnant."
"That's probably because I had a vasectomy back in 2009."