I agree follow your convictions. I don't know where people get the idea that thinking sexually about a girl is wrong, i'm quite sure it never says so in the Bible. How would people form relationships if they didn't have sexually feelings for eachother? The problem only begins, in my opinion when the thoughts act out of accordance with God's will for example if it is particulary peverse, selfish or un-loving. Sexual thoughts aren't necassarily lust, definitions of lust I have found include illicit sexual desire or intense sexual desire. I don't see how sexual feeling for a girl are illicit or immoral. I would say as long as loving and serving God remains your main focus, and you don't let the thoughts become your 'master' you should be fine, just be very careful and test yourself against the word of God to make sure you remain in faith. Personally I find masturbation to be a gateway into sin, but everyone is different and everyone doesn't fall into sin in the same ways.
1 Corinthians 6:12
Flee Sexual Immorality
12 “All things are lawful for me,” but not all things are helpful. “All things are lawful for me,” but I will not be dominated by anything.