If anything, he's horrible. You can't stay quiet about this if he confronted you about it, or even the fact that he accepted it. You're 15. You did nothing wrong. There are very strong laws against that, as no adult should EVER take advantage of a child. How did it "just happen"?
ill never turn him in were really close and what no one understands is that hes never been like a dad to me ever i have a dad i don't need another one! but were like best friends
But you're like a 15 year old girl and he's like a 30-year-old at the VERY minimum (I'm guessing here, maybe he's like 40). But here are the facts: 1. He had sex with a minor, you. That's illegal. That's probably the most frowned upon crime in the entire country. Even prisoners look down on statutory rape. 2. He's married to your mother, so not only did he betray her, but he made you betray her. 3. If it "just happened" with you and him, what's to say it can't "just happen" with him and the next friend you have sleep over? What if it "just happened" with someone who wasn't willing? 4. You didn't do anything wrong, he's the one that did wrong here. The only wrong thing that you can do is to stay quiet. Seriously, if what you said is true, then you absolutely have to tell somebody. It doesn't matter if he's your stepfather or your real father or an old friend or somebody that you just met, for anybody to have sex with a minor is a felony, and rightly so. Turn his ass in.
I change my answer then. That's a horrible thing to joke about. Some people have their lives fucked up pretty bad in situations like that. That'd be like me saying that I was in a car accident and was going to be paralyzed from the neck down, getting tons of sympathy, and then revealing it as a joke. So yes, you are horrible.