It's not about sex being dirty at all. It's just another area to look into where a person might have behaved selfishly, pridefully, hurtfully. I don't know about you, but I had a lot of resentments around my intimate relationships so in working my sex inventory it really helped to see where I was at fault.
That said, you'll get there in time. Find a sponsor and a BB first then go from there. The book explains it better than I can...