Upon release, the song received generally positive reviews from contemporary music critics, who praised its instrumentation, hooks and nostalgic lyrics, with some deeming it a highlight from Sex and The 1975.

In January 2012, The 1975 was formed by singer Matty Healy, drummer George Daniel, guitarist Adam Hann and bassist Ross MacDonald, who had played music together since 2002.

[13][14] The song has a length of three minutes and twenty-six seconds (3:26) and was written by the 1975 members George Daniel, Healy, Adam Hann and Ross MacDonald.

"[21] Elsewhere, Healy discusses relationship allegiances,[11] pondering the merits of receiving fellatio from a woman who already has a romantic partner with the lyrics: "And I'm not trying to stop you, love, but if we're going to do anything, we might as well just fuck.

[20] Reviewing the original version of the song, Brightest Young Things writer Bryce Rudow complimented Healy's vocals for their melodic quality while also praising the track's "catchy" production.

[17] Nothing but Hope and Passion's Norman Fleischer asserted "Sex" represents the EP's "obvious" standout and called it an "instant adolescent anthem".

[24] Reviewing the EP for Hit the Floor Magazine, Amy Jones declared "Sex" the set's standout song, commending the prominence of Healy's vocals and the track's subdued production, which she said allows the band to showcase their "incredible instrumental work".

[19] Andy Gill of The Independent and Digital Spy writer Lewis Corner declared "Sex" a highlight from The 1975, with the former describing it as an "infectious pop thrill".

[25][26] Pitchfork's Jayson Greene opined that the song is a "a perfect Jimmy Eat World mall-emo anthem" and deemed it the band's best track.

[27] Vulture included "Sex" in the publication's unranked list of the 10 Essential 1975 Songs; Larry Fitzmaurice compared it to Jimmy Eat World and LCD Soundsystem's "All My Friends" (2007) while writing that retrospectively, the song acts as a "a practical blueprint for the type of passionate, immediately catchy melodies" which Healy would later employ in the band's future releases.

[28] The Line of Best Fit's Laurence Day commented that "Sex" is evocative of the Killers' sound and complimented the updated production, which he deemed "fresher" than the original.

"[18] AllMusic writer Matt Collar said the song "drive[s] and climb[s] like the best anthemic '80s stadium rock" and commended its distillation of various influences into a single distinct sound, which he credits to delivering a "timeless" track.

The singer also praised the "passion" and "understanding" of Powell, whom he described as a "true collaborative spirit" that led them to film the visual in Los Angeles.

[13][44] Featuring sex scenes, smoking out of bongs, acid tripping and skinny dipping, the video was deemed NSFW by Sarah Bella of Music Feeds and the editorial staff of Variance.