This game is nice, but it has one major flaw: It feels like it's trying to tell two stories that don't quite fit.
On one hand, it tries to tell a story about fighting an alien invasion.
On the other hand, it tries to tell a "girl gets caught and fucked by aliens" story.
But sadly, the game can't reconcile both stories organically, leading in the largest issue with the game: the ACTUAL main character.
He feels like he doesn't belong at all in the whole "h-game" thing, since he never gets laid and his involvement with h-content doesn't extend further than "walks in on lewd things". As a result, he feels like a mix between the "shy harem protag who stutters around and stumbles into lewd situations he can never fully partake in and will likely die a virgin" and the "Light Novel protagonist who is inexplicably better than everyone around him, especially the ladies".
The other games do a better job at mixing the h-stuff with the rest of the plot and make sure to have a protagonist who partakes in both, at the very least.