But, O strange men!That can such sweet use make of what they hate,When saucy trusting of the cozen'd thoughtsDefiles the pitchy night: so lust doth playWith what it loathes for that which is away. (4.4.2)
When Helen thinks about her steamy hookup with Bertram, she wonders how it's possible that Bertram could have made such sweet love to a person that he hates (Helen). In the end, Helen chalks it up to plain old lust. This seems about right to us; Shakespeare scholar Harold Bloom agrees. He says that the bed trick is Shakespeare's way of making fun of men who don't really discriminate between sexual partners (source).