Imagine if this improbable thing happens:
In the polling upset of the century, all the German polls turn out to have the 2017 election backwards. Martin Schulz and the SPD crush Angela Merkel and the CDU/CSU in a landslide, winning so many seats that all he has to do is get the Greens onboard. He does this and becomes Chancellor in a Red/Green coalition.
Everyone is left scratching their heads wondering how German polls could be so ridiculously wrong. Unlike the Brexit and Trump cases, this would be a truly gargantuan polling error, rather than just pundits being overconfident in a slight lead that is actually within the margin of error. But somehow that's what happens.
Anyway, 10 months later, Schulz unexpectedly tweets a gif of himself hitting a golf ball and whacking Merkel. Is this tweet sexist?
No. It's pretty stupid and childish, in a manner unbecoming of a head of government. There's no need to post a crude gif in order to gloat over defeating the former incumbent party's leader, 10 months after you beat her. But it's a gif about a political victory over a formidable opponent who happened to be female, not an attack on women overall or something.
If Schulz had a history of making sexist remarks and behaving oafishly towards women, then he would be sexist, and the gif would be a stupid, childish remark by a leader also known to be sexist. But it wouldn't be sexist by itself.