Im confused, is this normal?

im 15 and i sometimes masterbate to gay porn, i dont understand why it turns me on though. ive had many girlfriends and a couple who i really really fell hard for. i watch straight porn too but after awhile it just seems boring, its wierd though because ive had alot of guy friends and ive never been attraced to a dude, ive never had a desire to kiss a guy or have sex with him but for some reason gay porn just still gets me going. im attraced to girls though definately, i get hard from looking at them. but ive imagined myself being with a guy and every time i do it doesnt feel like something i want at all, i just cant see myself with a guy.however when i see myself with a girl (one specific one in paticular) i can see a good relationship, ive never had any attraction to any guy ive met or feelings. but why do i still do this?