Why the heck does this game's upload have a 301.dll that needs to be renamed into 300 to work?
Anyway... If not for the opening being extremely bad, then this game would probably be rated pretty high. However, the whole "you have no actual choices to make but we're pretending that you do so we waste your time" thing was terrible. They could have just but in a long intro where what's her face gets stolen from the MC and THEN introduce the gameplay. instead of forcing you to sit through a bunch of "I'm with Remina, I won't do any night-visits." along with not really being able to do anything in general properly aside from dungeon diving.
Aside from that, pretty cliche virgin MC and equally cliche idiot child-hood friend who can't tell that she's being manipulated. But hey, you can turn her into a cum-toilet after that. That's pretty neat. Not many games let you do that. Not to mention that the whole girl swapping is pretty funny.
Also, this is pretty much how I'd expect the world to turn out, if not worst, if invincible monsters did show up and the only way to stop them was through magic that gets stronger through sex. Probably worse actually... The world-building is actually pretty decent... for a hentai game anyway.