Okay, I have a different view than the previous posters and have been thinking about this for days. I don't think it's appropriate for your step father to be taking an interest in your body when you dress. I'm not sure if he is just curious about your big bulge or if his interest is more the "filth" that DMW is refering to. I don't think it's a good idea to ignore, he may see that as an invitation. You can make friends with this man when your clothes are on! I would let him know that you prefer to dress alone and that his assistance or company won't be needed. I would aslo say, stand firm if he tries to persuade you it's no big deal. If it's no big deal, then he should be glad to give you some personal space, which is normal for you to expect at your age. The fact that you posted this tells me you are not comfortable with his attention, so just politely let him know, and see if it stops there. If it goes further after you ask him to stop, you'll know you were right.