In Splice the guy had zero qualms about fucking the creature, it was the woman who resisted and had to be raped.
Female human x male monster = romantic love story
Because women are whores who fantasize about being raped by psychopathic murderers.
Well, it's kinda both a romantic love story and a violent horror movie
Spring.
Because Hollywood wants to foster a positive female beastiality propaganda and increase the feminist image. It also implies that men are beasts and monsters who need to be tamed (and caged) or killed off.
If a male human has a relationship with a female monster, that implies that he's capeable of conquering the beast. And that's just way too a way too positive attribute to give to men these days.
While it's not that common in movies it's a popular trope in old novels.
The japanese already have an iron grip on the monster girl genere, so there is no point in trying.
Gamora and Peter Quill?
Men are commonly associated with monstrous qualities like ugliness, aggression, lack of self control, etc. Women love the idea that they get to fuck a guy who they can't control and then try and "fix" him (even though if they ever actually succeeded, they'd just get bored)
On the other hand, if there's one thing women hate, it's being told that they're out of control. People would probably complain that you're dehumanizing women by giving them visibly monstrous qualities. Besides,
nobody admits to wanting to fuck monstergirls besides weirdos on the internet
I have never once seen or even heard of a male human x female monster story unless you count the dinosaurs in Jurassic Park which are supposed to all be female, but that doesn't really count because they're not remotely anthropomorphic and capable of being romances yet