Those disgruntled supporters were voicing their disapproval at the American soccer star's decision to kneel during the US national anthem in support of Colin Kaepernick's silent protest against police brutality.
The Californian was the first white professional athlete to show solidarity with Kaepernick, the first to do so on an international stage, helping turn a ripple of discontent into a wave which swept across the country; from San Francisco to Seattle, from the NFL to the National Women's Soccer League, from elite athletes to college kids.
Two years on, Kaepernick no longer plays in the NFL but has become a global icon, Rapinoe captains her country and now stands with both feet planted for the 'Star Spangled Banner.' The sound of condemnation during soccer matches has dissipated.
Much has happened, yet the wheels of progress turn slowly, which is why Rapinoe continues to be vocal, continues to be at the forefront for equality. The 33-year-old Olympic champion is not one to hold back. She is, after all, the athlete who once described US President Donald Trump as a 'jerk but entertaining.'
'I would do it again in a heartbeat,' Rapinoe tells CNN Sport of her decision to take the knee in September 2016.
'I don't regret any of that. I still very very much stand behind Colin Kaepernick and what he was talking about and continues to talk about.'
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Rapinoe first kneeled before a league match for Seattle Reign, but did so again while representing the US in friendlies against Thailand and the Netherlands. Protesting in the National Women's Soccer League was one thing, doing so on international duty took it to another level.
There was outrage -- American Soccer Now held an online poll on whether Rapinoe should still be on the US team. Rapinoe, a World Cup winner, an Olympic gold medalist and an accumulator of 145 international caps for her country, had struck a nerve. But the winger never felt alone.
'Some didn't understand fully what exactly I was saying or what Colin was saying, or just hadn't dived into the conversation or topic but, on a personal level, I felt that support,' she says of her teammates.
In the spring of 2017 U.S. Soccer passed a new policy which required players to stand for the national anthem. Rapinoe has respected the new bylaw, but her beliefs have not changed.
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Asked what shocked her the most during that period, the player says: 'I was a little bit surprised in how much people purposely tried to change the conversation instead of looking at it honestly and looking at the numbers and the data and just seeing this isn't really up for debate.
'It is happening. It is just whether we're going to accept it and do something about it.
'That part of it was eye-opening and disappointing quite frankly, but I think it also opened up a lot of conversations and forced people to think about things in a different way, especially people whose lives aren't touched by police brutality.
'It forced people at least to start to think about it, whatever your views are on it. Having a conversation about it no matter how it goes is better than not having it.'
An openly gay athlete, Rapinoe came out publicly in 2012 and is dating WNBA's Seattle Storm's Sue Bird (they were the first openly gay couple to appear on the cover of ESPN's Body Issue). She has, in the past, said that gay rights, equal pay and racial inequality are intertwined.
What would she change about a divided US? 'I would like to see us as a country reckon with things more honestly.
'If we're still having the argument 'is there racism, is there sexism?' -- those things exist, we know they exist. In so far as we argue whether they exist, we're wasting time on the issue. I just wish we would just confront them more honestly so we can get onto the solutions.
'There's a lot of stubbornness and maybe disbelief in not wanting to have those conversations because they're difficult or maybe not knowing what to do about it. That's just stalling the process. The first step would be acknowledging these things exist, then we can get to the point where we can start making change.'
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Rapinoe was one of five players whose names were on the lawsuit filed against U.S. soccer's governing body in 2016 alleging wage discrimination. A new collective bargaining has since been signed and the team's fight seemed to inspire others -- Canada's soccer team reportedly sought advice on how to get maternity coverage into contracts, the US hockey team also asked for help.
Around the world, women's teams found their voices -- Nigeria's players held a sit-in at their hotel to demand unpaid salaries, Ireland's team threatened to strike, while Australia did and last year Norway won equal pay with their men's counterparts.
But there is still much to do, says Rapinoe, who is one of U.S. Women's three team captains.
'Everything is done at a snail's pace,' says the Seattle Reign veteran.
'On the one hand, progress -- however big or small -- is good and is being made, but I wouldn't say I'm happy with it.
'It seems an everyday tooth-and-nail battle to grow this game and get people to understand the potential of it -- getting investors and getting FIFA to care about it in the same way they care about the men's game.
'I will probably never be satisfied my whole life about where women's football is, but it's definitely growing quickly, and I've seen progress just in the time of my career.'
Over the next seven months, Rapinoe the social activist will center her life around becoming Rapinoe the two-time World Cup winner.
The US, the No.1 ranked team in the world, has qualified for next year's Women's World Cup in France with ease and come next September will be among the favorites to win the competition. Preparations have already begun, with victorious friendly wins against Portugal and Scotland hinting that it will be difficult to topple the defending champions.
'It's a box ticked off,' says Rapinoe of World Cup qualification.
'We're going to France in January for a couple of friendlies, narrowing our focus and putting final touches on things. From now on all focus is on France.
'With seven games it's going to be a long tournament and a decent amount of travel. We'll put it all together, but we've definitely got a lot more work ahead of us heading to France.'
Rapinoe describes herself as someone who leads by example and says she will aim to help the inexperienced players in the squad deal with the travails of tournament football.
'It takes laser focus [to win a World Cup],' she says. 'There are a lot of different variables -- travel, different games, different teams, good performances, bad performances, and you always have to have the big picture in mind.
'It's going to be hard to play seven games in a row that you feel amazing about and that you have great performances in. That's probably unlikely. You play a lot of different teams in a short amount of time and it's very taxing physically. It's not about each individual game, but the whole focus on winning the World Cup and what that takes.'
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There is also another World Cup further down the line of great importance to the US -- the men's tournament in 2026 which will jointly be hosted by the US, Mexico and Canada.
Rapinoe's former teammate Hope Solo was critical of the joint bid to host men's football's showpiece event, but but the Seattle Reign star is more diplomatic about the potential impact hosting the tournament could have on the game at all levels.
'It's pretty clear it'll have a tremendous impact on men's soccer, which is already growing,' she says. 'I hope it's going to have a great impact on women's soccer and youth soccer as well.
'There'll obviously be a lot of money injected into the system, but also just eyes and attention and, hopefully, raising the standard of all the leagues.
'Just imagining eight years from now, just imagining how much more popular soccer will be and the impact that it can have on soccer in this country -- hopefully it'll be tremendous across the board.'