Okay, that was ... emotional. NGL. It's also how I know I would react, to having to feel your love kissing someone else.
That said, here's my water bucket on that scene.
It makes no sense within the set up of the PLB. The initial premise of the PLB is women gave it to their lovers to make sure they stayed true and didn't cheat on them. If the lover did cheat, it turned into the Poisonous Heart Break Bug and could kill them both. XL knows this.
In fact, I surmise that scene way back in Episode 19 or 20, when XY rejected him and he left her to swim back to Dragon Bone Prison and Jing was STILL waiting there, and XL saw Jing carrying XY away. In that scene we see the PLB acting up, and XL forcing it to settle down. I think that was a moment, much like this one, where he felt his love turning to jealousy and heart break. But realizing what was happening, he controlled himself, remembered his feeling of true and pure love, and with spiritual power, forced the Heartbreak Bug to stay the Love Bug.
This scene, as emotional as it is, can't occur, without the PLB acting up again and becoming the Heartbreak Bug.
Also, this doesn't quite sound like XL to me. But more on that later.