Mansplaining is a sexist expression used instead of saying patronising or any other valid description, used to associate all men by implication with negative behaviour that can equally apply to either sex.
If you don’t think it’s a big deal just think of how many words gain a negative connotation when you add ‘man’ in front of them.
Child - Manchild
Flu - Manflu
Explain - Mansplain
Handle - Manhandle
Spread - Manspread
This wouldn’t be acceptable with any other characteristic. But our society finds it acceptable to demonise men in general. It would be nice if at least most men woke up and objected to this underlying animosity that’s permitted.