Ahh... Well, I don't know the story behind your life and how you ended up with a stepparent. And I'm not being mean here but I kinda don't want to know.
But it normally comes down to this: There are people out there that won't even consider a single (divorced, never married, or widowed) parent. I was one of those people and while I love the heck out of my daughter I ultimately wish I would have kept that attitude when I met my now wife. Because this is what happens: two people meet. They hit it off. One learns that the other has a child already. The childless parent is so interested in a relationship with the other that they decide to accept the child. The child is either cool with this and everyone is happy. Or the child wants to do anything in their power to drive the other person away.
Now this isn't to say that there aren't some really crappy stepparents out there that simply don't interact well with kids, because there are.
With the child acting out it puts a large strain on the relationship. Sometimes it will survive and other times it won't. It's usually the stepparent simply having enough and bowing out.
As for the "obese gluttonous" stepparent? They're in a relationship with your parent. If they only wanted to fuck her, well, that could be done without getting married. Matter of fact it could go on for years without the commitment of marriage. Before I did get married I slept with more than my fair share of single mothers. I had no intention on every committing to them. I simply didn't want the instant family. The financial burden. The possible drama. The inevitable: "you're not my father". Having to adapt to however you were raised (or lack of it). The fighting that may be going on with the bio-parents (if the father is somehow still in the picture). And everything else involved with raising a child, that I will remind you he had no input in creating. The easy thing for your stepfather to do was to "hit it and quit it". Then he wouldn't have to worry about some kid wishing for his death because he's having sex with mommy.
Now, am I saying that you need to respect this guy? NOPE! And while I can't say with any certainty that you're hostile to the guy, it sure does seem like you are. I'm pretty sure that whatever damage is done is done and can't be repaired. But I think that you'll find that if you're not hostile (if you are being hostile) that things may be a lot less strained. Not wishing someone's death is a start down the right path. And unless your mother is a very poor judgment of character or simply doesn't make correct choices (which I don't know, she is a single mother after all, so either could be true). I'd have to think that she thought this through before agreeing to marry the guy (unless it was for money, then the guy is the idiot in this case). You should have a bit of respect for that decision as well.
But again, I don't really care because I don't have to live in or near your house. But right now there are mixed families out there that are getting along and everyone is happy. And there are mixed families out there where the kid(s) are waking up and thinking how much they hate the fact that "this person is living is the house". And they are miserable. And that misery will eat at them and affect more than just their home life. What you decide to do with the pile of lemons that you've decided you've received is up to you.
And as I mentioned before: if you or your mother is in danger or being abused then yes you need to speak to someone about that. But if you're just upset, think that mom could have made a better choice, don't want anyone new in the house, want mommy and daddy back together, or any of the other things that fly around. Well, everything I said above does apply. When you get old enough: MOVE OUT. That will solve that. If you happen to be an adult: move out! If you don't live there: What the heck are you complaining about?!