His rhetoric question is also a bona fide straw man. I would feel uncomfortable yes and having sex with my mother would make me off the scale uncomfortable (both not just because she's dead). With another woman on the other hand, not so much, but still depending on who she is.
So, different people may have different feelings when it comes to a person or a picture thereof. Someone who only digs blondes or is gay may be like "meh"; no complaints were mentioned, if any. Other people get off on shoes.
Probably it was not too much too ask that the teacher had offered alternatives so as to increase the likelihood that an even greater number in the class like doing their assignment.
You can make a problem of anything. Look for a person. Why is it a man/woman? OK, one of each. Why is it a person of that skin color? Why is it a person of that age? Pffff.