People who think using insults rather than argument in politics is a good strategy, hone in on whatever they think is their opponent's most visible and vulnerable characteristics. That is often their looks.

In the case of Trump, it is his hair and coloring. Trudeau, his hair and youthful good looks. Harper, his bland, boring looks, his stiff demeanor.
In the case of many women, it is also often either that they are too good-looking, or not good looking enough. 

The troglodytes who send threats of any kind are just that, but of course they won't send rape threats to other males, that would impugn their own masculinity. Is there any chance of any of them actually carrying those out? Of course not, it's all bluster. 

When a male actually attacked a female politician, Pauline Marois, it was with a gun, not his p*nis, and not because she was female.

Remarkably, Merkel seems to have escaped such attacks, at least in English media, unless you consider her being called Mütter sexist...