I'm a boy, 15 years old. I'm bisexual, i never fall in love with men only girls but my problem is that i only know and want to have sex with a man and i only lust for men the other problem is when pretty girls flirt with me and try to have sex with me, my dick doesn't do erection.so if i don't get my penis erect then how can i have sex with girls? ... what i really want to know is how can i learn to lust for women and how can i get my penis to erect if the situation is with a female?

Additional Details:
i watch gay porn.