That is one suck situation to be in. I think you're doing the best with it given what you've got, which really isn't a whole lot to work with. I'm amazed you came out to your parents, un-supportive as they are, at such an early age and kept true to yourself for so long.
The thing that really sticks out to me the most is the amount of understanding you have for your parents' feelings, which seems to be showing itself in a sense of shame. I completely understand where such a thing comes from, I most definitely feel it, and it's the main thing keeping me from completely coming out to my parents. No matter what your step father may say, remember that you should not be ashamed. It does get easier when you move out for college, and neither you or your parents are in each others presence every day, or even months at a time. I hope things get better before then though, even if it's just by a little.