I would be veryyyy careful. Yes it is considered illegal to have sex, for it could be considered statatory rape, because you are not over 18 years old. Also, even though many couples have a large age gape, for some reason it is looked down upon younger generations (for the maturity difference.. but that is still REALLY hypocritcal.. just saying). So be aware that any law enforcement or parent may have this view as well. Also, the parents would be able to charge the man with staturatory rape or just an inappropriate relationship with a minor (or something along the lines, I'm not sure about the actual legal term), without the minor's consent because the girl is under 18 years old.
Hope that helps.