I had the students look at semen under the microscope
Are you serious? Why did the students look at that?
"I asked them if they ever struggled to learn things when they were in high school. Most of them said yes, but when I asked them what they learned how to do it, they usually pointed out their hands or feet. They would say "I learnt how to walk", or "I learnt how to tie my shoes." Then I showed them semen under the microscope and asked them what they saw. I told them, "The sperm are like little puppies", and everyone laughed.
Stuff like this can be expected in a college class. But you should forewarn the students beforehand that things like this might be in the syllabus.
Why do they have to be warned, what happened to the freedom of speech? We are the professor in the classroom.
I can understand, but I think you should inform the students when you're going to do something invasive. Besides questions would come up about whose it is and where it was gotten from. You should be more respectful of the others.