There are several moments for Kurt that help him realize that, he isn't like other boys.
That he doesn't like the soft curves of the fairer sex.
That although beautiful he never wanted to touch, or hold, or kiss.
He never wanted to tell a girl he loved her more than he ever loved anyone else, on one knee, with a ring that would tie him to her and her to him for as long as they both should love, and care, for each other, until death do us part.
He thought about it of course.
Thought about the girls in his class and thought about how he was supposed to go, 'would you like to go out with me?' or how he was supposed to think about what they look like 'under' their clothes, instead of how he asked 'would you like to go shopping with me?' or how he thought of how they looked like with a certain 'dress' or how 'flattering the shoes would be to their legs' but not 'for him', no because he didn't think about girls 'like that' no because he thought about 'boys' 'like that'.
Sometimes he wanted to want girls like that.
When on Friday nights and his few friends have plans with their boyfriends, or girlfriends and he was left all alone to watch t.v. his dad snoring on the couch next to him, or sitting in his room feeling the loneliness so much worse when his 'dad' had a 'date', then he would want to want a girl in his arms, he even wanted to want to ask a girl out but then he would just hurt her when he came out as gay and what if she wanted more he wouldn't be able give her more because he didn't really want her.
He just wanted to.
When he was three he had asked for a pair of sensible heels. Not because he was gay, no that wasn't the reason not at all. It's just that what his mom said.
"Oh Kurt, never wear heels over three inches. Always get sensible heels."
He decided that he would get sensible heels just to please his mom.
Because his mom wanted sensibles so he should want sensible heels to impress his mom.
Just like his mom put on make up so he should put on make up.
(When his dad found him after he had scribbled lip stick all over his face his father had been stunned. Until Kurt had said 'look i'm just like mommy!' then it made more sense. Yet his dad still suspected something.)
When he was 7 he was pushed and called that word that 'f' word that he would be labeled with even when he didn't understand (although even when he did he still didn't understand how people could be so cruel, although, he had at some point accepted it.)
When he had asked what it meant his father had told him never to say that word again and his mother said it was when a boy liked other boys.
He had asked if that meant he was one because he liked other boys.
While his father's face had went red his mother had said no, if you like boys you are gay, that word was just a horribly mean word to say it.
That was the first time he had heard the word gay too.
(Then he had asked what it was called if you like girls because girls were so much nicer than boys. His father had started laughing and hs mother chuckled. Now he knew it was called being straight, what about 90% of the population was.)
Then in highschool he had met Finn who was handsome enough and the nicest boy besides Artie to him. He had started to develop what he thought was feeling, he was pretty sure that he was gay at that point. (Mercedes had offered to go out with him and while appealing he realized he didn't really want to. Then he had come out to her and his dad and they had been so supportive.)
Then he had started the whole thing with Brittany. (She had told him from the beginning that she just wanted to make out with him for her make out record.) He thought that this would be his chance to test the waters and maybe their is a chance he could be bi. (He didn't think it was possible for him to be straight. He was just too... gay.") Then they had started to make out, it was nice, but very awkward and she had to take the lead the whole time. The next day he went back to being himself and apologized to Brittany. She didn't really mind as long as she could say she turned the capital G gay kid straight. (He had started laughing but agreed.)
Then Karofsky had kissed him in the locker room. It was rushed, forced, sloppy, and gross. It was also Kurt's first kiss from a boy. It had made Kurt doubt things, because frankly, Kurt had preferred kissing Brittany. He decided he would wait and see when he kissed someone he was in love with.
Then he had met with Blaine who had bought him coffee and lunch and he had thought that maybe he had already found that person, and after all the ups and downs when he had finally kissed Kurt.
Kurt just knew.