During adolescence young people are still 'allowed' to experiment, but after this most people know their sexual preference and behave accordingly.
Heterosexuality is the basis of our society. Most people are or feel heterosexual and the heterosexual family is seen as the ideal way of living. Because of this we distinguish between people who can and who can't satisfy this norm.
Sometimes we don't know how to treat people with any other sexual preference than the average. This is because we find it hard to imagine the feelings of a person with another sexual preference.
Parents of homosexual or bisexual children may be disappointed because they have other expectations of their child or they are embarrassed by the child's behavior. This can lead to discrimination, depressed feelings, fights, aggression, breaking friendships, etc.
A tip on how to react to homosexuality: Think about exactly the same relations, expect that it was heterosexual. How would you react to it in that case? Then react to the relations the same way you would if it had been heterosexual.