In Dramatica, "theme" is not so much about subject matter but what you have to say ABOUT the subject matter. In your friend's story, "Sexual Love vs. Spiritual Love" would be the thematic subject matter. You could say all sorts of things about that subject matter, including using Morality vs. Self-Interest as you suggest. Without knowing your friend's opinion of her subject matter, it's difficult to know what would work best for her. For the sake of example, here are some possibilities that I've picked randomly:
Truth vs. Falsehood -- "Love that grows from spiritual strength is true love while love based on physical desires fosters lies and deceptions"; or its inverse, "Love based on the physical aspects of human beings is more true to human nature than the delusions inspired by purportedly lofty, intangible, spiritual love."
Expediency vs. Need -- "Love based on sexual convenience is far more satisfying than love inspired by some spiritual need"; or its inverse, "Spiritual love is often far more convenient than the needs satisfied by sexual lovemaking."
All you need to do is pick any of the variations of dynamic pairs and use them to provide commentary on the subject matter at hand.