What if the picture on screen was a woman saying 'I want to be judged on my merits not my gender' and the guys were saying 'Nice tits' out loud and laughing? Would that be sexism?

I'd unequivocally say yes and, while inarguably less offensive, the dongle joke was still similarly sexist.

The game of what is considered sexually "offensive" or sexist gets very complex when you start considering sexuality outside the "norms". Suddenly, traditional "sexism" and the assumption that everyone is a straight male and straight female seems laughably archaic in a discussion of modern ethics.

What people who don't fall within the norm are mainly concerned with is ridicule, condemnation, threats, abuse and violence from those considered 'normal.' Whatever part of your identity is non normative doesn't significantly change that.

The main thing that changes is that the degree of disdain/disgust/superiority towards your group tends to determine how severe the abuse is; leading to statistics such as that 1 out of every 8 black transgender men end up being murdered.

Tragically enough nerds are also definitely on that spectrum yet somehow the abuse they suffer often doesn't translate into empathy for those even further down the food chain.

How so? As I understand it the joke wasn't directed at the picture on the screen or anybody in particular.