Question: If your partner cheats on you, who would you 'attack' first; your partner or the third person?
I realized most answers would be the third person.
Another question: Why?
You should talk to your partner first because for all you know, that third person maybe was not informed about your partner's status. That third person might not know about your partner's relationship status but your partner sure as hell knows about his/hers.
And girls, if there are girls who are flirting with your boyfriend. Please don't say boys are being boys by 'entertaining' those girls. That's just like "Hey boyfriend, you can 'talk' to those girls because that's what boys do". No. Don't give excuses for your boyfriend.
Unless if you have a proof that your partner did not spark any fireworks and that third person has no boundary with your partner, then you should go have that talk with the third person. Other than that, ask your partner to set some boundaries.
Because it takes two to tango.