This is called bisexuality. Just because someone has a same sex experience, it does not mean a person is gay or lesbian. Many teens experiment and have these relations. It can be a natural part of learning about your sexuality. No one knows why someone is homosexual.
Some people believe it is something determined before birth.
It is usually not a choice. It is important to remember that being gay or lesbian is not something doctors think is abnormal. Our culture does not do much to support gay and lesbian youth. Sometimes, teens dealing with these issues turn to drugs or alcohol to cope with the pain they feel from society. Some people have thoughts of suicide.