Well...no, not really. It's like this, everyone has a dark side, but it can be expressed as art. A lot of people don't like Marylin Manson because they say he's a devil worshiper and stuff like that, but in reality he's very intelligent and sophisticated.
Anyway, you can show art that's unusual and stands out, but deep down you could be a completely different person then the art you represent. Let me put it this way, in some of my novels I put in all kinds of heavy language, swearing and sexual themes at times. In real life, I don't swear at all and rarely talk about sexual themes. Just because you have a piece of art that may seem dark in real life, doesn't mean that you as a person are the same. Just like actors, most actors in real life are nothing like the characters they portray, think about that.
You like to draw that kind of stuff. There isn't really anything wrong with that. I'm thinking your siblings are reacting the way they are because it's not something they're used to seeing, and it's not something they personally would like. It was also probably one hell of a surprise.
I think they could have chosen their words towards you better. I suppose you could try to explain to them what you said about you not being a perv or a freak, and tell them that what they said to you hurt you, even if your tastes in drawing did make them uncomfortable.
The only way drawing erotic art would make you any of those things would be if you were having sex with it. Other than that, I'll echo what other people have said and say that you can draw whatever you want to draw and who cares what other people think of it.
There's nothing wrong with drawing erotic art, I'm thinking of starting drawing some myself, it's a great way to practice human forms and proportion. Your picture is definitely not porn or whatever your sister makes out it is, she sounds very immature to call you names for drawing a picture.