Personally speaking, it's a fear of being able to get and maintain an erection. It's not because i'm not attracted to the woman, or because i have ED, it's purely because of nerves. A lot of guys can suffer from erection issues when they feel incredibly nervous, stressed, or anxious and that's something i think and worry about considerably. Worse still, i fear that if that should happen, my partner would laugh at me for it. And like most other guys, i worry about my size and whether i'm big enough or not (both erect and non-erect, even though one of those is the only one that matters) and whether she would laugh at me if she was dissatisfied with my size, even if it was considered average or above average. I also fear she would compare me to other guys, and i would hate to be a 'failure' in her eyes. I would worry about whether or not i'm doing it right (touching in her in all the right places) and be able to keep her aroused. I have pretty low self esteem and anxiety issues, which really doesn't help things. All of that at once is a hell of a lot to think about, and it's something that would plague my mind before it even got to the undressing stage. I suppose since i don't have a typical "alpha male" well toned Russel Crowe body, i also fear i'm unattractive in her eyes. That i might not be "masculine" enough, or she will find me unattractive. I tend to have a cynical and slightly hostile view on life and people, so i'd nearly always jump to the conclusion my partner would laugh at me rather than be understanding and try to put my mind at ease. I pretty much tell myself that she just wouldn't have the patience for my kind of issues.