The broadcasting regulator launched an investigation after eight viewers complained that the encounter between contestants Terry Walsh and Emma-Jane Woodham was broadcast too soon after the 9pm cut-off.
A spokesman for Ofcom said: "This programme included sexual material shortly after the watershed. We're investigating whether the transition to more adult content was unduly abrupt."
ITV2 aired the footage, which was not explicit, on June 30.
Walsh had already been seen getting close to two other women in the house, while Woodham was the ex-girlfriend of another contestant.
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The show was at the centre of an earlier controversy when Miss Great Britain Zara Holland was stripped of her title after she had sex on the show.
The 20-year-old model, from North Ferriby, East Yorkshire, was shown getting intimate with 24-year-old scaffolder Alex Bowen.
She was told off camera that she had lost her title and initially chose to remain on the ITV2 show. She left days later when her mother became ill.
The Miss Great Britain organisation said its decision was taken with ''deep regret''.
A statement said: ''We pride ourselves on promoting the positivity of pageants in modern society and this includes the promotion of a strong, positive female role model in our winners.
''The feedback we have received from pageant insiders and members of the general public is such that we cannot promote Zara as a positive role model moving forward.
''We wholly understand that everyone makes mistakes, but Zara, as an ambassador for Miss Great Britain, simply did not uphold the responsibility expected of the title.''
The organisation later tweeted a message to say: ''To be clear, we have no problem at all with sex - it is perfectly natural. We simply can't condone what happened on national TV.''