Boy, isn't that the question... I would argue that I was already questioning/curious about penises/men/males when my first experience happened, and it was so mind-blowing and earth-shattering that it definitely played a huge role in my trajectory/development. i.e. I don't think there's any way after the experience I had bottoming, being knotted, having the longest, strongest most intense orgasm of my life up to that point for, like, 8-10 straight minutes, that I would continue to think of myself as straight.
I've only had women for romantic partners/significant others, and I'm 'out' to friends to varying degrees, but I remember the thought I had in the moment at the time: "Oh... I get the word/concept 'bitch' in a totally new way now," and it was an identity/label that really took hold and stuck with me. So I can't imagine what man would have the experience/reaction I did and continue to consider himself straight. I might have to browse through the 7 pages of responses here and see if there's an answer to that in here somewhere...