Exactly, they'll have sex, but virtually always with people their age. When a 25 year old has sex with a 17 years old it's almost pure sexualization. Sure, the older person could be a social outcast or be in the autistic spectrum, but even those are terrible excuses.

When you think it's alright to have sex with a child (doesn't matter how mature you believe they are at 17) you are directly contributing to their increased sexualization. There's a reason it's well documented that very young children who experience sexual abuse are far more likely to be sexual deviants and do these kinds of things like seek out older men who take advantage of them purely by pretending it's acceptable to have a sexual relationship.

Yes, they'll have sex regardless, but when an adult, or adults, give into the wishes of an inexperinced child they're constantly sexualizing her and giving into immature desires that are far more likely to lead that person down a destructive path. Not always true, but it increases the chances a large amount.