'Twas so; but this, all pleasures fancies be (5)
You know how when you see the movie of a book you've already read, the actors' faces completely supersede your own imagination? Suddenly Jennifer Lawrence is Katniss Everdeen. The same goes for true love versus sex. Compared to the sincerity and electricity of the speaker's current relationship, all previous sex seems to melt away.
If ever any beauty I did see,Which I desired, and got, 'twas but a dream of thee (6-7)
He continues this thought in the final two lines, promising his lover: all those earlier babes he scored? Those were as insubstantial as a dream. Now he's awakened to the real thing. The takeaway? Sex isn't real unless there's love too.