Go and talk to the girl instead of asking your bf to lose his face.

Maybe you really not good enough for him enough that why he dare to challenge you to let you know, you not the only girl.

With 10 year relationship, lower down your pride and get over it with your pride instead. In city life, lust seduction is hard to resist. Office lust is easy and quick to get it.

So you may want to change and improve your behaviour too. It take two hand to clap...

If u love him enough - i m sure you can forgive him, likewise vice versa.