Stay with me here.
I was always curious about Charles Widmore and his hatred for Desmond. I get that he thinks no one is good enough for his daughter, but most father's think that about guys their daughters are dating. So I came up with two theories; one's real easy, the other is kinda hard to explain.
The first is that Desmond is Widmore son. We know from one of Ben's flashbacks that Widmore had a child with, I think the term used was "Outsider". Widmore used his considerable wealth to track Desmond over the years, and when he showed up asking for Widmore's permission to marry Penny, he declined.
My other theory is that Desmond gets back to the Island eventually, but now in 2004. He somehow gets time shifted to when Widmore was on the Island. During Widmore's time on the Island, Des tells him about his wife and son. Then, Desmond somehow dies. When Widmore gets off the Island and has Penny, he realizes that Penny is the woman Des marries. Since he knows Des will die, he wants to stop Des from marrying Penny spare her the pain of losing Des.