A question for agnostics/atheists?

My parents are both adamant Christians, and due to my ytical nature, I'm just prone to atheism. We argue (respectfully) about a lot of things, and even though this difference creates a barrier in some ways, it's starting to evince in more areas as I grow older. I will always respect their beliefs, but I feel as of late that this gesture is not mutual. For example: My uncle is forty something years old, and it's obvious on several occasions that he's slept with his girlfriend of three years. What seems perfectly normal to me, my parents refer to as 'insulting to the family, especially to my grandparents'. I don't see how a grown man having a private life is insulting, and when I mentioned that I would sleep with someone I was in a long term relationship with (but not formally married to) I got the same reaction. My parents believe that by sleeping with someone, I would be dishonoring them, when I reminded them that once I am an adult, it's really not even any of their concern. It all seems so entirely unreasonable to me.