"The Untamed" by Amat Escalante is the movie you're looking for:
A couple in a troubled marriage locate a meteorite, initiating an encounter with a mysterious creature. Their lives are turned upside down by the discovery of the creature, which is a source of both pleasure and destruction.

La Bette(The Beast).Screwed up French movie with a woman being chased through woods by some kind of werewolf/apeman.Not full on sex but I remember the creature had a prosthetic erect penis and there were all these close up ejaculation shots.

Near the end of Prometheus the grown up squid baby (Trilobite) face rapes or face hugs the Engineer who gives birth to the proto Xeno (Deacon) obviously influenced by the much smaller face huggers seen in Alien and its progeny. Hell Boy 2 had a squid creature face rape which led to the death of an auctioneer.

The latest version is Splice, which isn't tentacled but a part-human/part-animal creature.

Well, they are not all monsters, and most of them don't have tentacles, but so far these are the movies I've found where some kind of supernatural rape happens:
Evil Dead
Entity
Inseminoid
Breeders
The beast within
Humanoids from the Deep
Terror Within
Hybrid
Galaxy of Terror
Rosemary's Baby
Demon Seed
Demon Rage
Hollow Man
Possession (1981)
Witchery (1988)
Progeny
Xtro
Astronaut's Wife

Not sex but some action in Jacob's Ladder..dance scene in party

Garth Merenghi's Dark Place has giant Eyeball sex.

Drop Dead Fred.

There's a weird hallucination scene in 'Jacob's Ladder' which, while not a full-on sex scene, involves sexualized contact between a woman and a tentacled demon creature.

In 'Naked Lunch' a guy gets raped by a giant insect. Basically anything by Cronenburg. You'd also have luck with movies by Ken Russel such as 'Gothic.' There's some bizarre mutant sex in 'Class of Nuke'em High' where a guy has a monster for a penis. In 'The Church' a demon has sex with a chick. And even though you said no Japanese 'Tetsuo: The Iron Man' has got some pretty funky sex scenes with a dick that is like a tentacle, same in 'A Snake of June.'
Also the artwork of H.R. Giger, who designed 'Alien' and 'Species.' (Anyone who knows his work knows the trademark long alien head is penis inspired.)

Implied in Galaxy Quest.

I was thinking of:
Sometimes they come back again (1996)
with a very hot Hilary Swank!

There's some of that in
Species 2
(1998) as well, between the slimy "human beings" who turn into alien creatures!

How about that scene in
Evil Dead
where the girl gets beep by a vine? That do anything for ya?

Yeah I've seen Society (1989) - pretty funny film.
Well all I could come up with is the Species Series of films, Inseminoid, and Breeders, but I guess its true Possession is unique.

Society, Im not quite sure whats going on there but it`s definately some unforgettable weird sh!t.
Out of light cometh darkness.

Or maybe
The Dream of the Fisherman's Wife?

"zoom back camera"

Of its kind I believe this is rather unique, however I seem to recall there's a painting called 'Animal Lust' which was probably inspired by this film. You might find it image search