i never used to fantasize about men before. im 15 and straight. recently i fantasized about a man unconsiuly. now im worrying that i might be gay. please replyy i need help

Why are things so real the first time they’re in my mind and then when I think about it later it’s easier for me to be like wtf?? I was watching a movie earlier and the young girl had developed more in the chest area than the last movie and I felt the desire to check her out so I did. Then later I let myself imagine her having sex and I liked it. But now looking back I’m like ew. The boys in the movie have also developed as the movie went on and I couldn’t help but think that in their real life they’ve probably woken up to boners and s*men and stuff. And looking back it’s just ugh. Idk if it’s sexual relevance but I genuinely let myself indulge in these thoughts and groinal responses and I remember thinking to myself I don’t want to be attracted to little kids and how do I stop myself (everyone has attractive qualities so in younger boys I see man like qualities). Idk I need help. I wouldn’t type this out if I truly believe I was messed up but I’m still scared

can i turn gay? and suddenly start liking gay and men sexual parts even though i never liked them before i scared i will start doing it and lose my attraction to females

i'm positive i was attracted to women before this got a thought when i was high thought really really deeply into and changed my life now im 24/7 scared im gay ive always been attracted to girls but early in my sexual life where im at ive always got with girls and seemed a little disapointed after would love help and to hear past experiences