Twitter dominated by tongue-in-cheek #HeterosexualPrideDay

What appears to be a tongue-in-cheek social media movement to mark June 29 as a day to celebrate heterosexual pride has become one of the day's top online trends.

The hashtag HeterosexualPrideDay was trending on Facebook and Twitter Wednesday, with hundreds of thousands of tweets and posts reacting to the idea.

Some tweets took a serious tone, and some were joking or sarcastic, but most expressed outrage. One of the top responses comes from British comedian Dapper Laughs, who rhetorically asked on Twitter , "What's next, #BreathingAirDay?" Others have flooded the hashtag with gay pride memes.

The social media trend came at the end of LGBT Pride Month, when cities across the U.S. hold parades and celebrations. This year's festivities were tempered by the massacre at an Orlando gay nightclub.

President Barack Obama proclaimed June as pride month this year and recently designated the site around New York City's Stonewall Inn as the first national monument to gay rights.

The Twitter user who first tweeted the hashtag declined to comment to The Associated Press. It's not clear if the account's intent was serious or meant as satire.

Christian ministry Focus on The Family and The National Organization For Marriage declined to comment on the hashtag.

Gay rights group, the Human Rights Campaign, did not immediately respond to a request for comment.