'His Passionate Bride', an edition of the regular series of short animated films 'Three Minute Wonder', received 21 complaints from viewers for being too sexually explicit before the watershed.
The animation, broadcast at 7.55pm in November last year, featured women's genitalia, scenes of sexual intercourse, oral sex and accompanying sound effects.
Channel 4 responded to the watchdog by saying that the drawings were "sparsely drawn" and so people would divorce them from feelings of reality.
It also noted that pre-broadcast it warned audiences that the programme would take "a wry look at intimate adult relationships".
Ofcom found the animation contained clear indication of sexual acts including the accompanying sound effects and female genitalia. It also said that Channel 4's pre-broadcast announcement did not fully explain the programme's content.
The media regulator found the programme in breach of family viewing policy and the Watershed; and in breach of the sex and nudity section of the programme code.
Separately, ITV's daytime show 'This Morning' also found itself under investigation by the regulator after a sex counsellor showed detailed images of models in various sexual positions and advised women how to achieve orgasm.
Ofcom found the material unsuitable for daytime broadcast, when large numbers of children could be watching, but not in breach of the code because no sexual movement was shown in the pictures.
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