I'm 16, a high school kid, and not married or in a relationship. I really like this one girl and want to be with her. I love my Lord and Savior Jesus Christ more than anything ever in the entire universe, and would not want to take even the slightest chance of not being able to live with him in the kingdom of heaven. I have trained myself to not want sex, which means I have no feelings of lust toward anyone. I was just wondering that if I touch this girl's breast or butt, is it a sin if I have no feelings of lust toward her whatsoever? I don't want to have sex with her, but based off your answer, i may want to or not want to touch her.
Your words are not consistent. Do you go up to random girls and touch their breasts or bottoms? No? Then there is a reason why you want to do it with your girlfriend that doesn't apply to strangers. Lust isn't having an erection. It isn't your ultimate intentions. Lust is justifying doing things that should not be done.
I would contend that you want to touch your girlfriend in sexual ways because of the feelings that it produces in you and her. But those feelings won't be the endpoint. Each time it will creep a little bit further, trying to sustain that same level of excitement. Eventually you'll go further than you intended because you'll be too caught up in the moment to think about where things are heading.
"Now concerning the things of which you wrote to me: It is good for a man not to touch a woman" (I Corinthians 7:1).