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There are plenty of instances where a 15-year-old girl [or thereabouts] and an unrelated adult male authority figure have sex and it doesn't appear to harm the girl. At least, not at first. People react differently to these things. I know a woman who said she "seduced" her junior high science teacher when she was 14 and looked back upon those memories very fondly. Seems rather sickening to me, but not to her. Most of these relationships are, inherently, predatory and injurious. If it involves someone like a teacher it becomes even more problematic. If one or two teachers are allowed to get away with this behavior on the grounds that no one was hurt [and maybe -- in those cases, no one was], it'll spread to other teachers and there WILL be a lot of young people hurt.
We have a general rule against these sorts of relationships in our culture. Like most general rules, it isn't perfect, but it exists for a good reason. The alternative would be far, far worse.