Sir Lancelot and Queen Guenevere have a passionate affair even though she is married to King Arthur, but all we see is Guenevere entering Lancelot's bedroom (he has no shirt on), and Lancelot and Guenevere kissing while apparently bathing (partially clothed) in a lake. We never see any actual sex or nudity, nor do we ever hear any heavy breathing or moaning. In another scene, Lancelot sneaks into Guenevere's bedroom while Arthur is away, but both are fully clothed. They sing "I Loved You Once In Silence". Lancelot then leads her to the bed, but before they can do anything, they are surprised by the evil Mordred, who is trying to destroy Arthur's kingdom.
Mordred is King Arthur's illegitimate son - he was conceived by another woman who had a one-night stand with Arthur before he ever met Guenevere. Mordred describes what happened.
The song "The Lusty Month of May" innocently implies sex, although Vanessa Redgrave performs it with very suggestive vocal inflections.