Excuse me, but in that type of situation, the 25 year-old ADULT isn't the one who is the victim. She's 15 years old. People that young do unwise things. You even said yourself about a predicament you were in - you've obviously done stupid stuff, she's done it, kids that young will do so. It's scripture.
But it's obvious your "help" is coloured by your resentment that she didn't want to date you exclusively. What you did was humiliating and demeaning to her.
If someone she's fooling around with is 25 years-old, that's not the time to debase and embarrass her. That's when it's time to go tell her parents what she's doing. She may be "stupid" for flirting around with someone that old, but the point remains that HE, as the adult, has the responsibility to refrain from hitting on minors her age, and his failure to is highly illegal (well, at least if it goes sexual).
Rather than using these instances to try and salve your pride because she didn't want to date you, actually HELP her and tell someone who is in a real position to stop her from making a mistake. Otherwise, you have no right to claim you "love" her. Loving someone means accepting them, faults and all, and doing things in their best interest. Your public outburst was not that.