There's a rather simple explanation for this. Politics has become engrained into many parts of our culture, to the point where at times it's indistinguishable from personality, or outlook. Something as simple as a disagreement about tax laws wouldn't normally endanger a relationship. But something more life changing, and effects the every day, would—and that's increasingly what politics are effecting and related too. When someone says 'Trump Voter' you don't think "Person who voted for Trump and I know nothing else about them."

You automatically assume many non political things including stuff all the way from whether they only practice missionary sex, to whether they only drive pickup trucks.

When someone says "I wouldn't marry a trump voter" they aren't necessarily saying they'd never marry someone of a different political affiliation, they're saying that wouldn't marry the stereotypical persona of a "Trump Voter."

Much like when I say "Karen" I'm not just talking about anyone with the name Karen.