This is a very painful topic. I believe the cause is my husband's depression. I feel much better about myself since I went for therapy. Before that, I was sure that is was my fault. He has an ever-ready excuse: the kids will hear (if awake), the kids will wake up (if they are asleep, the kids may want to use the bathroom (next to our bedroom)and the biggest excuse of all, the ever-exciting TV program.
I can go into greater detail if you e-mail me, as I am not comfortable discussing certain things on a forum.