As far as I understand it, sinning is a matter of will. It has to be something that you do of your own free will otherwise you are not culpable. So when you willfully transgress God's commandments, you are sinning against Him. Now having a wet dream is not something that you can help doing. It is out of your control, and God sees that. But fornication is something that you do. Either with a woman or on your own, it is a grave sin either way since it perverts the sexual desire and function God has given us as a gift to share with our husband/wife as He intended for us. When you masturbate, you are using this gift for selfish end. And you are probably not having chaste thoughts when doing it, and Jesus said that anyone who sees a woman and has impure thoughts about her has as good as committed adultery (I'm paraphrasing here but you get the point). When you have sex with a person that is not your spouse, same thing goes. Except this time you are using each other for your each own selves' pleasure. That is not what God wants, it is not the natural purpose for which he created sex. There is also the fact that you are also leading/partcipating with another person into sin thereby multiplying the consequences. About the virginity thing, I'm not sure. It's a difficult thing to define in this day and age, but I don't think someone who masturbates is as innocent as someone who doesn't. So if you say virginity = chastity, they aren't virgins. Idk really tho. I hope this addressed your questions, sorry for the lengthy response.