For those debating whether this is legal or not as the younger party is 17 this could be deemed to be Child Sexual Exploitation.
The below extract is from the Metropolitan Police website.
Sexual exploitation of children and young people under 18 involves exploitative situations, contexts and relationships where the young person (or third person/s) receive ‘something’ (e.g. food, accommodation, drugs, alcohol, cigarettes, affection, gifts, money) as a result of them performing and/or another or others performing on them, sexual activities.
Child Sexual Exploitation can occur through the use of technology without the child’s immediate recognition; for example being persuaded to post images on the internet/ mobile phones without immediate payment or gain.
In all cases, those exploiting the child/ young person have power over them by virtue of their age/ gender/ intellect/ physical strength/ economic situation or other resources.
Violence, coercion and intimidation are common, involvement in exploitative relationships being characterised in the main by the child or young person’s limited availability of choice as a result of their social, economic or emotional vulnerability. (ACPO National Child Sexual Exploitation Communications Strategy)