Their love for each other was so pure and vulnerable that they knew no other way to part. Only a society as perverse and degraded as ours could so associate love with sex that we could not separate the two. Hollywood has defiled us. In almost any movie you see, if two characters in the movie love each other, they always end up sleeping together before the movie is over. By this repition in our lives, we begin to associate love with sex, but the society that David and Jonathan were raised in never would have made such a connection between sex and love.
Love exists independent of sex.
You could not. David and Jonathan shared the kind of love that knew no defilement. Their love was so deep and sincere that Jonathan actually preferred David over his own family. Their love was so pure and trusting that David actually treasured it more than the love of his wives. This is why David was called a man after God's own heart - because he loved unconditionally, not just Jonathan, but many people, and when he loved someone, no matter what they did, he loved them until the very end.