You are apparently referring to a "teenager" below the legal age, else you wouldn't characterize it with the word "wrong". You don't specify if the other one having that "relationship" is a teenager also, but I suspect you are referring to an older person.
Furthermore, persons of that age are unable to make reasonable decisions on their own because they lack life experience to gauge consequences. Such are easily deceived and manipulated, and should therefore be given space to mature. Just because they have fully working adult bodies, that doesn't mean they have adult thinking.
If by "love" you mean actual love in which one puts their neighbor's wellbeing above one's own, one would be loving that person enough to give that person space to mature so they don't make a decision they later regret and can never undo.
Since that doesn't seem to be the case with your question, then the other person of which you are speaking loves himself/herself more than the "teenager", seeking one's own pleasure above all else selfishly.That is not "love". That's hatred to put oneself above the wellbeing of another in order to receive self-pleasure.
Genuine love will sacrifice one's own pleasures so that the other is kept inviolate when most vulnerable.