I think the answer is simple. They treat women differently from men because they look for people's weak spots to attack, and being female is something they consider a weak spot, i.e. they are sexually vulnerable in a way men are not without impugning the troll's own sexuality.

When men do attack male politicians using sex and gender as a weak spot, it is by casting doubt on his masculinity, i.e. by calling Justin Trudeau  "Justine",  or those who brought up Harper's supposed "cowardice" in "hiding in a closet" in a recurrent, macho insult.

The only proper response imo is to ignore these and insist on only responding to actual rational arguments,  instead of taking the bait and responding in kind, which only gives them the satisfaction of knowing they upset you.