Upon release, the EP received generally mixed reviews from contemporary music critics, who praised the exploration of various genres but felt the record suffered from a lack of cohesion.

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

[4] Writing for Hit the Floor Magazine, Amy Jones said that in contrast to Facedown, Sex contains a "slightly darker [and] more foreboding" tone,[12] while Paste writer Shaina Pearlman observed "equal parts ethereal and synth pop" throughout the record.

Koellner wrote that "Sex" marks the EP's sonic shift from a "companionless headphone" record to the "centre of a party", characterised by "traditional" guitar-heavy compositions.

[14] DIY's Martyn Young shared this opinion and said the EP is "arbitrar[ily] split between the first two overtly electronic hazy pop songs and the second two "more straightforward traditional guitar tracks".

[12][20] Thematically, Healy said the track was inspired by the dissolution of his social group, noting it represents "that moment when you realise, [']Oh, that was a bit of a waste of time--but fuck it, I'm actually just as happy as I was before so no harm done[']".

[9] Awarding Sex a score of 9 out of 10, Amy Jones commended the EP's musical diversity and its development of the 1975's signature sound, asserting the record shows promise for the band's debut album.

[13] Norman Fleischer of Nothing but Hope and Passion praised the variety of styles present on the EP and deemed the title track an "obvious hit single", writing that the band "stick[s] up to the expectations" set by fellow Manchester musicians.

[16] Martyn Young awarded Sex three out of five stars; asserting that while the record showed "a great deal of promise"—with the exception of "You", which he opined ends the EP on a disappointing note—he struggled to understand "exactly who [t]he 1975 are", owing to the band's "wildly fluctuating" sound.

However, she felt it was "apparent that The 1975 are still searching for their sound" and criticised the transition from the first half of the record to the second, comparing it to "an accidental click of the shuffle button".

Despite praising the "dynamic" sound of "You", she called Healy a nuisance and criticised the record's overtly sexual nature, saying the singer "whines melodramatically about all kinds of things, from the irrelevant to the trivial".