It may not be a 'crime' but it still feels immoral IMO - a 16 year old is still very much developing, emotionally, socially, morally AND neurologically. They're more likely to have limited knowledge of relationships and social skills, and be more easily manipulated. They won't have the experience to recognise unhealthy behaviour and abusive treatment, and someone older, eg: in their 30s, will have MUCH more experience in developing relationship skills, they will be much more independent and will likely have resources the 16 year old will not have access too, giving a HUGE power imbalance to the relationship.