I don't think the gauge only measures the horny level but more like a person's level of frustration and satisfaction overall. Like, he was frustrated or unsatisfied all the time even though he had sex with his "best friend" because it didn't mean they were together-together and they were more like fubu (friends with benefits) than a romantic couple. He wanted more and therefore, his lust was never satisfied and his gauge was always full and on the verge of cracking/exploding. And then he thought his bf did the same thing with his two other friends -- offerred to masturbate together or have a threesome -- and his jealousy completely snapped his rationality. That means even though the gauge measures how frustrated and unsatisfied a person is, it also indicates if the person is still holding on to their rationality or self-control. The moment it breaks, the person snaps and loses all their rationality and self-control.

did he just- rape- hi- im out (⊙…⊙ )