How did you come to learn...

I agree with Sarah. The homosexual half of the question is, no offense Paul, self-explanatory and thus doesn't need to be asked. Everyone is born through a heterosexual process, and even if you argue the transgender pregnant men they were still women at one point, and thus the notion that everyone has heterosexual biological birth parents holds water. In this sense, like Sarah said, you'd have to have been raised by wolves or something since birth to not know of the existence of heterosexual humans.