Sexuality is a powerful force, which, as you say, is difficult to forgo. But since it is such a powerful force, we need to be careful so that it does not cause harm.

I agree with your own opinion that your behaviour can cause trouble and is not right. It is also not right against your mother, nor against your stepfather. Your stepfather is the husband of your mother. What would she say if she knew what you are doing? I do not think that you really want to take the consequences of arousing your stepfather so much that he cannot resist you. But I understand that this is not easy for you. If you are not able to leave home and live somewhere else, you should immediately stop these sexual games with your stepfather, and find people outside your home to play with.