If someone served you a drink which was spiked, you would be less liable for driving under the influence.
If someone gave you a drug without your knowledge, you would not be responsible for taking an illegal drug.
He was misled. From his perspective a woman a similar age to him, out and about at a time when children are not out and about, had sex with him.
He had sex with a twelve year old. But not because he did anything young people across the country don't do all the time.
If we don't want this to happen again, we need to challenge the culture of having sex with people we don't know, not punish a nineteen year old for behaving in a culturally appropriate way.