Harems: Vns and real-life
The idea of a harem is a very old one. In fact, there is literally no culture (prior to Christianity) that, sometime in its past, did not accept the idea of a harem in one form or another.
However, it was never a simple system of one man simply marrying as many women as he wanted. In some places, it was a mark of familial wealth. In order to avoid long-term difficulties, a wealthy individual would take numerous wives, produce numerous sons, and split that wealth amongst those sons in order to prevent his fellow villagers or townsmen from turning on his family at some future point. In others, it was a mark of political power. An Emperor would take numerous wives, both to form alliances with his nobles and foreign countries and in order to ensure the continuance of his dynasty (in most Imperial dynasties, it was accepted that there would be vicious palace politics that would kill off numerous potential heirs). In old China, it was accepted that a man would take a primary wife and many concubines if he had the wealth to do so. The primary wife would be the mistress of the family, whereas the others would help administer the husband's estates under her direction.
So why did the harem system go out of fashion? It didn't, really. Most men of power and wealth have mistresses, even today, and the attitude of most 'old money' families is one of resignation toward this kind of behavior. Of course, religiously and legally those living in the mostly Christian west can only have a single wife, but the tacit approval of wealthy and powerful men taking mistresses is a work-around common to almost all monogamist cultures.
Now, for the morality of it... extract religion and hypocritical social mores from the mix, and you basically have three golden standards by which the idea of a harem might be condemned or approved:
1. Are the men/women involved satisfied with the situation and well-treated?
2. Does the male or female holder of the harem have the wealth to support his/her spouses and any resulting children?
3. Are the resulting children happy and healthy?
That's real life. VNs do it a bit differently. Because VNs tend to avoid dirty details (no, I don't mean H, I mean real-life style drama) and ignore inconvenient realities, the average VN probably wouldn't be able to fulfill those standards in the long run. This is because most VN protagonists don't have the capability or strength of will to both manage the inevitable internal conflicts of a harem (herding cats) and bring in enough money to support a large family. For this reason, most VN harems just don't qualify in my eyes as being a realistic idea.
That said, there are VNs that actually consider the realities and make it seem like a realistic choice... but most don't manage it, lol. I especially have to rofl at harem nukige and moege.
Now, for my personal viewpoint... I basically think that if you can manage to fulfill the 'golden rules' above, there is no reason why I should care if you have multiple wives/lovers. On the other hand, if you can't afford to have a wife and kids, you shouldn't. I hate optimists.