"...are minors really legally allowed to have sex with each other?"
In most states, yes. For example, New Jersey sets the age of consent at 16, and has provisions to allow even a 19 year old to have sex with minors, provided it not exceed four years difference, I believe. If you are really interested in state laws regarding ages of consent and the legality of gay sex, ageofconsent.com is a good resource.