Having doubts about my sexuality

I'm a 16 year old male and recently I've been worrying if I'm really as straight as I thought I am.As long as I can remember, I've always liked girls. My first crush was a girl, my first erection was from a girl and when I thought of eventually having a girlfriend and getting married it used to make me feel immensely happy. I've never looked at men (gay or straight) and felt arousal - until recently.I was watching a show on TV and calmly thought "Hey, that man looks really good looking." For some reason this took me by surprise as it was the first time I'd ever thought like that. I decided the next day to try and masturbate to gay thoughts (I don't really masturbate to porn, only images and erotica) and I'll admit it felt great, which also really surprised me as I'd never done such a thing before, so I've been worrying for almost a week about whether or not I'm gay.I'd like to point out that I'm not at all worried about how my parents or sisters will react - they're all very open minded and my parents have expressed numerous times that if any of us were gay, they'd be totally fine with it.So yeah, any help is gladly welcomed.