I need to be sexier/prettier...

Why is that my husband can get hard and get off to watching porn or looking at another naked woman's body, but not mine? I play with myself in front of him like he likes, I tell him I'm down for anything, but when we try to do anything he just gets bored and goes soft. Why can't I be his fantasy and the girl he wants to get off to? Why can't I be more sexy or beautiful or whatever it is he is seeing in them?