The sweetest kind of love is first love: that shy, little crush you had when you were, perhaps, thirteen. (Or ten, if you were somebody in the world of Mary-Kate and Ashley) The most agonizing kind of love is lost love: maybe a lover that had passed on, or the somebody whom you never ended up with. It's cruel and painful, it's effects are meant to last.
However, there is a kind of love that is sweet, sweet like a nightingale's song; painful, like ripping the heart apart. It is none other than that forbidden love: the apple on the tree of Eden. Love is forbidden by the society, by rules set by people, by us. From some long long time ago, when God (or evolution)'s actions resulted in us, we were meant to be falling in love with the opposite gender, we were meant to be falling in love with people unbound by blood.
Then, the society also made way for people who have to be pure and clean, untainted by the lures of bodily attraction in order to serve god. The clergymen (and nuns) are a common feature in sensual manga/anime, building the attraction based on the forbidden love. Cosplay maids in Akihabara sometimes dress up as nuns to allow their masters a taste of that forbidden fruit.
Similary, 07 Ghost plays on not just the typical Shonen action to appeal to its shonen audience, but also intense character development and that guilt factor to grab the adulation of its shojo population.
As Kamiru said, in 07 Ghost, "The Bishops are SEXUALLY ATTRACTIVE