Reduce = become smaller. If you reduce a man, he becomes less important. Similar to a physical object. Up in arms means many angry people, protesting or revolting.

Hi Hazem, Reducing is in regards to men or a man. A man is a complex being with emotions and feelings. When you say that joke, you are reducing that man to only what he can do in terms of sex. When someone is up in arms it means that they are protesting angrily, or are very angry about something. I hope this helps. Aleks