For one group of protesters trying to gather support for ballot initiatives that would bar transgender girls and women from school sports, the Portland Thorns game last week was an opportunity to draw attention to their cause.

They chose a big Friday game for the women’s pro soccer team, one the Thorns needed to win to make the playoffs. They sat in seats close to the field, just behind one of the goals.

And most important: The general admission seats — first come, first served — were next to the section reserved for the Rose City Riveters, an official fan group of the Thorns.

Based in Portland, Ore., one of the most liberal U.S. cities, the Riveters are known across the National Women’s Soccer League for their political activism, passion and rowdiness. Like the Riveters, the pro league is also known for its progressive values on LGBTQ+ issues.

So it was no surprise that the group of two dozen protesters evoked ire from the Thorns fans.…