I wanna nail the "Oh shit, I like pussy" moment.
Well, I'm a guy, straight like you, so I think that you can also benefit from this. I mean, to build the character correctly, just knowing how it is for the persons isn't enough ; you also need to be able to translate this into your own perception of the world. Else you'll write the cliché, not the true thing.
So, I'll start by telling you how my wife and I guessed that one of my (20 years younger than me) cousin was lesbian. You'll see, it's as simple as
@Avaron1974 said.
She was 11/12 years old at this time, and a girl like any other girls this age with one very small and almost imperceptible, still obvious once you've seen it, difference : She was acting with boys her age like she should have acted with girls, while she looked at girls the way she should have looked at boys, and vice versa.
It was not a tomboy thing, she had this phase later, around her 17/18 but it didn't lasted long, she was trying to figure her place in the world and tried this. No, at this time it was just an hormone thing. Once teens hit puberty, they feel attracted by, well, it's not always "the opposite sex" so let's call it "loving sex". But like it's something new, straight girls are all, "Er... A boy, so grosssss", while straight boys do the same with girls. They don't avoid them, they just feel them as gross and tend to stay more often with people of the "none gross" sex. And there's the glances... You're with your buddies, and come a person that you find attractive. At this age you don't understand that it's desire that you feel, but this desire is already clearly visible in your eyes when you look at the person.
You can take the problem from the opposite side. When and how have you discovered that you are straight ? You didn't, right ? It was something natural for you, and it's the same for lesbians. Except that you are part of the majority, so you were going with the flow and as everyone (including people you'll never met in your life) expected you to go. But this exception is just a society thing. A lesbian don't naturally feel that she's different, it's the society that make her feel it, not her mind/body/sexuality. Like
@Avaron1974 implied, she felt the exact same than anyone else until the moment girls around her started to talk about boys. And still after this, I don't think she effectively felt different, she just know that she have this difference ; but here it's more a question of personality and environment (familial and societal), some lesbians have hard time dealing with this difference.
It's generally not something you discover. People who say they discovered that they are lesbians are mostly people who were confused and thought that, because they are girls, they mandatory must love boys. But generally what's hidden behind this, "I discovered it when I was X years old", is what Avaron1974 said about her first and only kiss with a boy. Some kind of, "once I had a straight sexual (rarely more than a kiss or holding hands) interaction, and it felt so wrong". More than a discovery, I tend to see this as the proof needed to confirm that, "fuck the society, I'm a girl, but it's girls that I like".
I mean... Let's say that you don't know how to "fix a car" (personally I don't), but the fuck, you're a man, you must know that ! Look around you. Looks the ads, looks the movies, men know how to fix a car ; each days, thousand of men around the world are fixing their own car. So, well, one day your car need to be fixed, and like you are a man, like you are expected to know how to fix it, you try... And now you know for sure, you can't fix a car. Whatever the society, the movies, the ads, the news, your friends and your family can say, you are a man, yes, but you don't know how to fix a car.
And it's exactly the same thing when I talk about the "proof needed". Mostly you can't go against society's pressure without trying first. But it's not a discovery, you know yourself, you already knew that you can't fix a car, you just needed to be sure that it's effectively true ; and this is more true for lesbians/gays, because at this time they still are children, worse they are teenagers, they are sure of nothing and in a perpetual search of themselves.
As for trans, it's something totally different. My best friend, the brother I chose, is F2M, and I helped him deal with his past.
While lesbians (globally) tend to past through this like it was nothing, for trans it's way more visible, and it can lead to a real and strong suffering if they are repressed in a way or another ; a suffering that will stuck with them forever and change their personality. They don't "behave like boys" (it's a
shortcut to summarize what I said above and
just a shortcut), they are boys.
Lesbians can wear dresses, skirts, be girly and all ; not all, but globally you don't dress according to your sexuality. In some place there's a mandatory dress code, and lesbians deal with it. They can dislike it, but they still comply ; "My job need that I wear dresses, I need my job, I'll complain a lot about it by I'll wear dresses".
For a F2M trans it's something different. They are a man, but they have a fucking pair of boobs, which imply that anyone will treat them like a woman, call them "miss", and all. They'll always do as much as they can to looks like the man they are despite their boobs ; and once they can, they'll pass through surgery to remove those boobs. Note that when I say "looks like" it's not limited to clothes. They'll do there best to dress, act, talk, react, as a man is expected to dress, act, talk, react ; some do it so much that they become the caricature of a man. Their job need them to wear a dress ? For the large majority they'll quit this job. And I know one person that did it even while at this time having a job was a "life or death" situation ; hopefully he find one a month later and had good friends that helped him pass through this month.
But like for lesbians, it's not something you discover, it's something you feel, and it start as a very young age. From there come the suffering. If the parents are stupid (voluntarily or not), they'll enforce the gender stereotype ; "no my daughter, you can't have a fire truck for your birthday, you're a girl, you'll have a doll". It's a negation of what they are, and it imply the feeling that their parents don't love him. All this happening at the young age where you build yourself. Think about what you felt the day your parents said, "no, you can't go to your best friend birthday party, we have something else planned this day", or something like this ; you really wanted to do "this", and your parents said "no". Then, report it to every single instant of your life. All your youth you really wanted to be a boy, because you
are a boy, and your family (starting by your parents), as well as the society (you're a child, you have teachers, you have schoolmates), say "no, you are a girl". You don't pass through this without suffering, you don't leave these times of your life without a strong anger stuck deep inside you.
That's why so many trans tend to over do it and be aggressive. They have nothing against you, but if you call
her instead of calling
him, you'll trigger a storm ; the storm of all those years where every single second of their life people told him, "no, you're a girl". A storm that tend to disappear once their birth sex become invisible ; simply because he's now not mingled with a girl and, more, he's finally seen as the boy he is. But the anger is still here, and it need time and a lot of loving for it to fade.
Obviously, not all trans, are like this.
M2F tend to be a little more peaceful. They had the same suffering youth, but once they are on their own, they can become a woman ; the lack of boobs isn't a problem, there's girls with tits so small that you aren't sure that they are girls, and you can always stuff a bra and wear it. Same for F2M with small boobs, that can more easily be seen as a man. But both have this anger and suffering. Some deal more easily with it than others.
There's also, too few, trans that have a comprehensive family. They still suffer from their interaction outside of the family. But they are loved at home. Here they can be the boy they really are, and it help them a lot to survive those years and have less anger stuck inside them.
Oh, last thing regarding trans. People tend to see them as homosexuals ; you are F2M because you are lesbian. It's stupid. Yes, the majority of the F2M love girls, but it's normal when you think about it. They
are, really, totally and without possible doubt, male. And when you are male, you have around 80% chance to be straight and so to be attracted by female. Even if they don't transition to the body they should have had at birth, and so stay in a female body, they'll never see themselves as lesbians but as straight.
Yes, they have female body. Yes, they fuck girls. But they are straight men in a female body fucking with... and here come the complication... Because straight women will not have sex with another woman, a F2M end having a sexual relationship with either lesbians, bisexual, or pansexual, but really exceptionally with straight woman.
Note that if the relationship is with a lesbian, generally it will not last this long. Sexual orientation isn't just about the body and love isn't just sex. A F2M tend to be a little too manly for a lesbian ; and I don't say it as in the cliché lesbian looking and acting like lumberjack.
As for M2F, sexually it's even more complicated. As for F2M not being lesbians, they aren't gays. They are female that love male, but don't expect them to let you fuck their ass. It's not all women that agree to this and among them not this much that effectively have pleasure doing it.