Even though I was getting crushes on girls ever since I was a kid, up until I was in my 20s I basically thought that that was just something all girls think about and I was still straight because I liked guys too and have only dated men (while I was growing up bisexuality/homosexuality wasn't really something that people casually talked about in my country). Then, one of my guy friends started dating this one girl. After a month or so of them dating, he wanted her to meet his friends so we all went partying for the weekend. Considering she didnt know any of his friends at that point, she brought her sister along...and as far as I was concerned, her sister was the most perfect woman I've ever laid eyes upon. Eventually, we started chatting...I discovered she was a really awesome person too. One thing led to another and we hooked up. Dated for a year or so after that. So, that's when I realized that I may in fact not be as straight as I wanted to make myself believe.

Tl; dr: Hooked up with my friend's girlfriend's hot sister