First of all man, we've got kids on this site, all right? I'm no mod or anything but you didn't have to say something like in that first paragraph, that's pretty messed up.
I'm not saying looking at a girl = lust. But if you say, "She's hot, man, I'd like to kiss her," I'd say that's lust. I don't look at guys and want to kiss them, I don't look at my mom and want to kiss her (not in a romantic sense I mean), because I don't have lust for them.
I think ultimately you're mixing up what I'm saying. I'm not saying that kissing a girl you're not married to is bad. I'm not saying being intimate with a girl you're dating is bad. What I'm saying is if you're kissing this girl and cuddling with her and in the back of your mind going "I'm not gonna marry this girl," that's bad.
If I date a girl, and have every intention of marrying her, and I kiss her, and cuddle with her, and whatnot...and after a few months we decide, hey, it isn't working out, we should end the relationship now, I haven't done anything wrong. I was genuinely caring about her, and wanted to be with her, and it just happened that she was wrong.
That's fine. What I'm saying is wrong is never having the intention of marrying her in the first place, not having the genuine care for the girl, that is when the sin comes in. Saying, "I want to be with this girl because I want to have my desires of having someone to cuddle fulfilled," that's when it becomes lust.
Sin is an act, and if i were to take that jesus scripture in the context of what you say and apply it to another law, like stealing then that means that whenever i see something and the thought of me stealing it comes into my head happens. Because i thought of it, i've already sinned.
I'll have to correct your theology here. That's the point Jesus was making is that even THINKING of committing sin is the same as doing it. God judges us by our intentions, that's the point Jesus was making. It's the same reason why He said that if you look at your brother and say "I hate you" you're already guilty of murder. That's pretty strict, to say that hating someone is the equivalent of killing them. But that's the standard God holds us to. It's not our acts that define us. Ever seen people give money to charity? What if, in the back of their mind they're going, "Man, I don't like doing this but I want everyone to think I'm a good person."
That's why you can't say it's our acts that define us as Christians. It's our hearts. It's our intentions. That's how God judges us. Sin isn't an act, it's a defiance of God. And even the thought of defying God, is sin.