Well my first time was a founding moment in the development of who I am today. You see, at 15 I never had any success with figuring out girls. Perhaps as a young man I expected to much, but in absence of any real relationship probability, my best friend and I embarked upon our first sexual relationship...a homosexual one.
Three decades later and I really don't think I've figured out women yet, even though I've been married to one for almost two decades. I guess I am truly bisexual, however I've always considered myself mostly gay which goes back to my very first experiences.
Did it change me? It did reinforce that I truly don't care what sex my partner is...that it is about the person and how that person fulfills me as a whole.