The battle system is interesting, still trying to feel that one out for how its balance will work- the game seems to encourage having "releations" with all the heroines, monogamy be damned.
That said, it also appears that whatever heroine you take into the dungeons (and you can only take one) heavily influences things- you take on the elemental atribute of the heroine- and kids born from the gal perform better when she is along. The heroine also seems to influence which classes would best come from her - (a strenth based heroine would produce good physical attackers, magic based heroine would produce better mage kids). These factors seem to encourage sticking with the same heroine and her, um, kids.
but of course, if you were facing a water boss, you sure as heck don't want the fire heroine in your party making you incredibly weak to the enemies attacks, so I guess thats how they encourage enjoying the whole herem. heh.