When Omar Mateen bombarded a gay nightclub in Orlando, Florida, with bullets earlier this month, his father quickly identified his son’s possible motive.

Seddique Mateen recalled his son getting upset after seeing two men kiss while with his family several months ago. Though he said he did not understand why Omar Mateen would enter a gay club and open fire, killing 49 people and injuring dozens more, he pointed out Omar’s disgust.

"They were kissing each other and touching each other and he said, 'Look at that. In front of my son they are doing that,'" Seddique Mateen recalled to NBC News last week. "And then we were in the men's bathroom and men were kissing each other.”

Mateen’s hostility toward seeing two men show affection played out in an extreme way, but in the US, it’s emblematic of a barrier the LGBTQ community faces. Even with support for marriage equality reaching record highs at 55 percent, there’s still an “ick” factor that comes when seeing two people — specifically, two men — show affection.

A study published in the American Sociological Review in 2014 showed heterosexual people are willing to support formal rights for same-sex couples, such as inheritance rights, hospital visitation access, and marriage equality. But when it comes to seeing same-sex couples kiss and hold hands in public, straight men (and to a lesser extent, straight women) think LGBTQ people should curb the public display of affection.

This sentiment isn’t new, of course, which is why kissing as a sign of protest is nothing new for gay activism. Writer and activist K. M. Soehnlein told the New York Times that he used to stage kiss-in protests in the 1980s to confront homophobia that fed into the lack of action on combating the HIV/AIDS epidemic.

Those kiss-ins reemerged in the wake of the attack through a new form of protest: social media. Last week, photos flooded timelines aptly accompanied by the hashtags, #TwoMenKissing, and #TwoWomenKissing. Not only were the lip-locked photos a sign of protest against the homophobia that may have inspired the shooting, but they were also a sign of love.

Still, it’s not just straight people who balk when two men show their love. Gay male participants in the study were “significantly less approving of the gay couple kissing on the cheek and French kissing, compared to the heterosexual couple” doing the same things in public, the journal article reads.

But Long Doan, co-author of the American Sociological Review study, said there could be several reasons for that.

“Some of that is attributed to internalized homophobia, and how society socializes people,” he said. “If you grow up being socialized to believe same-sex couples are less loving or less-than in some way, it’s hard to escape that.”

Some have claimed Mateen himself used gay hookup apps and visited Pulse nightclub on multiple occasions. (Investigators are looking into this point, but it has not been confirmed if Mateen was gay or bisexual). One man claims to have been Mateen’s lover, and told Spanish-language outlet Univision that the shooter was jealous of those who could be open with their sexuality. The media outlet hasn’t been able to confirm the alleged lover’s claims.

Doan added, though, that the discrepancy of why same-sex couples aren’t as encouraging of PDAs, may be “due to fears of crime or of attack.”

Soehnlein echoed this worry: “One stray kiss or a moment of my husband and I holding hands runs the risk of someone saying something awful or being attacked.”

This all turns into a Catch-22 for LGBTQ people who are in same-sex relationships. Doan co-authored a follow-up study that found many people still don’t see relationships between two men as being as loving as heterosexual relationships, or relationships between two women.

That disparity, Doan said, “was actually linked to their support for rights and benefits for same-sex marriage. So it creates this disconnect. How do you prove love? How do you show other people that you’re in love if you can’t show affection?”

While this study shows there’s still a long way to go when it comes to general acceptance of LGBTQ people, Doan says it also shows how far America’s attitudes have come. In writing the journal article, Doan and his colleagues cited a study conducted around 2003, which asked Americans about their attitudes on LGBTQ people and their rights.

“People would be uncomfortable with even saying the word ‘gay’ on the phone,” Doan said. “They would just whisper it. But once it hit the national dialog, people were more used to the idea, and they [became] able to talk about it.”