I think you have it right on the nail Blackstone.

"What is a biblical definition of lust? And does fantasy qualify?"

but of course this is a leagal question and we are no longer under law.
As fallen creatures sin infects everything we do. Living under grace means learning to put aside the sinful desires of our heart/flesh from out of a relationship with our heavenly Father in order to live God honouring lives.
How can a boylover do that since the Father is not going to flick an "off" switch on our sexuality.

I pray like this:
Father, I'm a boylover, I strongly desire sex with boys, how can I live a life that honours You when I have this desire? Surely sexing boys would be dishonouring... but there is so much energy within me that is sexual. I take responsibility for my own sexual desire... I release it through masturbation... have mercy. Help me to walk in Your love.

Blessings
Cat.