What's going on with the Leonard and Penny in this Big Bang conflict scene?

Seems Leonard is getting all worked up about Penny having a male musician friend sleep over at her apartment:

What interpretations is Leonard making on the situation? What is his perception of things?

What topical, relational and interest goals are perceived to be at stake for both Leonard and Penny?

Which emotions (and why?) escalate from the conflict and the thwarted goals /interests/needs?

How could these characters behaved or communicated differently to understand each other?