@SotiCoto most people don't fall in love with their fav chars (like them for other reasons), so they don't get jealous. I think the best comparison would be how you can become a fan of someone (be that an idol, royal family, or celebrity), and as a result be happy when they find love and build a family.
Though judging from what I read in manga (surely an accurate description of reality! (at least I
hope it is as obviously flawed of a representation as I assume it is)), this does not apply to jp culture, as there the boys get jealous and wants to kill
any person who gets a girlfriend at all, no matter whether they are in love with the girl or not. And if an idol has a boyfriend, she is somehow sullied and not fit to sing and be idolized?