The indoctrination for homosexuals is very thorough. Good training is the hallmark of a successful organization.

Trial and error (lots of error over the last 40+ years of being sexually active). And there actually was this one time in the back of the bus . . .

And I've got a burning desire to learn, read lots of books, taken many classes and workshops, speaking at colleges for over 16 years about BDSM and I've had the extreme pleasure of running the Center for Sex Positive Culture for last 11 years. This all gives me a wealth of knowledge and experience. And I still have tons to learn and plan on keeping at it for another 40 years at least.

Allena

I'm a pervert.

I read all of Dan Savage's archives.

This one time? At band camp?

I acquired my sexual education from Judy Blume.

Man, that bitch is freaky.

I taught human sexuality in college. (No lie.)

And since then, I have maintained a healthy collection of sex manuals, education materials, and light instruction manuals.

1. internet porn
2. relationships with open communication
3. casual sex

My double major in college was Gender Studies. Also, read all Dan Savage's archives.
Sex is just a fun thing to learn about/talk about/think about/have.
Plus, Q-land seems to attract pretty open minded, expressive, knowledgeable people.

I'm quite fond of the Good Vibrations Guide to Sex. And other books.

Well I think there is some comfirmation bias going on but the real answer is I just find sexuality and biology and psychology super interesting and I've been self educating myself about sex since I was 13. I would actually go to the library and sneak sex education books meant for adults. My mothers way of having the talk was to leave "Our Bodies Ourselves" in my bedroom. I've also taken college level sexuality courses.

Just being open about talking about sexuality, and realizing that sexuality is a sort of science, will allow you to absorb a lot of useful information.

I have an intimate relationship with Ceres, Frejya, Dionysis and Aphrodite. I make the right sacrifices for information and then I wait for the gods to take hold of me and use me as their vessel to spread their relationship and secksuality related truthes...

Either that or through hypersexual trial and error mixed with a healthy sense of curiosity...

More than anything: Orgasms are powerful and addictive motivators (to learning, for example). ;)

Multiple partners lends itself well to giving one experience to draw from, as do plain old 'tree rings'. Parenting adds a layer. Having a sexually 'abnormal' childhood adds a layer, and having gone through counseling/therapy etc helps a small degree. I was sexually active at an early age, and that led me down a path of addiction, which adds another layer. Reading the works of Pepper Schwartz, Dan Savage, Drew Pinskie, Anne Hooper, et al doesn't hurt. Being bisexual can, arguably, double one's exposure to a few things. 'Lab' research is of course the best method for one to, ahem, 'come by' your information as you say.
and HELLS Yes, the field trip bus backseat was indeed instructional. Several times, from 6th grade up, really. Got my first taste of genderbending back there, as well as various "firsts" involving 2nd & 3rd base.
My earliest instruction was cute little Jean, the girl next door, showing me her naked body and begging for a kiss after showing me her dad's impressive Playboy collection - we were 6. My best early instruction was age 11, when my parental unit gave me that cartoonish book called "What's Happening To Me".