18+ modern day harem I couldn't resist posting here (LOL this manga is just too funny if you can filter out all the explicit scenes and focus on the story - but only if you can...)
Purimu no Nikki

harem is a place that is forbidden for all male that has reach puberty to enter.. it could be a house inside a house with it's own bedroom, living room, bath room, reading room, work room, etc, everything that women might need; or it could also be a one/more room in the hose where no male allowed; or it could also be not a house like a female only bathe-house, the female only part of the onsen, female locker room, or that female only train car etc
if your house have a harem then that's where your wife, daughters, mother, grandmas, aunts, sisters, anyone who are female do their activities (study, teaching, crafting, gossiping or just a simple afternoon tea) on the house without any male to disturb them... young male whom hasn't reach puberty is allowed to enter (but not allowed to sleep there.. if i'm not mistaken).. female can go in and out as they pleased, male can only stand at the door and talk to someone from there.. (and can only get in with permission.. if i'm not mistaken)
this is mean:
1. if u have a complete harem your wife doesn't always have to sleep with you..
2. no it's not necessary mean you got to have more then one wife to have a harem
3. you have to be rich to have a fully packed harem in your house (complete with hot water pool, library and everything).. your house prolly be a castle at this point