well...I think that sex education includes sociology, besides biology (anatomy). And if you talk about sociology, you have to talk necessarily about the relationships between the two genders.
Gays and lesbians, I have learned from a reliable source, are known to live in the same world as anyone else. There are persistent rumors that the two genders are present in that world as well.
The rumors about children who never noticed that straight men and women exist or that failed to understand what non-homosexual affectiveness is remain unconfirmed.