You do not understand because you are not someone who has been discriminated against simply because you are part of a group that ignorant people choose to demean. Consider this:
I was an employee at a company and was a top performer. I was the go-to person for everyone else in the department - if they didn't know how to do something, they came to me for answers. When a more senior position opened up, everyone "knew" I was going to get the promotion and corresponding raise. I didn't. Who did? A guy who spent at least half of every day making personal calls, who literally did less than a tenth the amount of work I did, plus the quality on what little work he did was shabby. No one in the department respected him, and everyone was shocked that he received the promotion.
What did the department manager tell me? "I know you do more and better work than this guy, and you are extremely respected by everyone both inside and outside this department - you deserve and earned this promotion. But he has a family to raise, so we are giving it to him." Translation: He's a male and you're not. What about me? I was the sole bread-winner for my family, and the department manager knew it. Regardless, what did that have to do with the job? Nothing. This was sexism, pure and simple.
Try putting yourself in my place, and in the place of any other woman/black/jew/etc. who has been discriminated against - how would YOU feel if a woman had been promoted and given a raise over you, simply because she had no penis?
Like racism, sexism is not a joke. It is a serious offense, and is inappropriate to joke about, as is anything intended to demean a group of people simply because they are members of that group.