They've been hurt before, and so they've forgotten how to trust others.
"I'm really not good in relationships. It wasn't always that way. It's just that the first two women I was with cheated on me with a friend (different friend), both of whom told me about it later. I guess I developed a mindset that if I couldn't trust women and they were likely to do it again in the future, I might as well not deprive myself."