Thanks for the interest, Ajoxer! :) I'll try to offer a few observations, if I can... where to begin, where to begin...
Ah, yes, the The Evil Dead's tree rape. Now, I was born without womanly parts, so, unlike Lilac, I can't suggest I'd know exactly how it would feel to have a branch jammed in down there. I was born with plenty of bodily orifices, though - all over the place, in fact! - and I can say that jamming a dry stick into any part of your body, never mind a sensitive area, isn't especially pleasant. Still... well, they are "evil" dead, aren't they? I wouldn't expect them to wine and dine poor Cheryl...
By the way, I have a crush on Ellen Sandweiss, so that's one big reason she was included here. I can't get enough of her schnoz. Of more substance is the fact that she comes across as a really easy-going geek girl, which is nice, especially considering her claim to fame.
Next, Galaxy of Terror. Lilac gives a good impression of what goes on in the famous rape scene. Basically, the movie concerns some space explorers who are being preyed upon by their nightmares; one of the explorers is claustrophobic, for example, and thus is crushed in a narrow corridor. Dameia's worst fears include rape and worms, so... she gets raped by a worm. The scene is included here partially because it's the inspiration for galaxy-of-terror.com, which is arguably the largest monster rape website around. It's a good scene, too, in a relative sense: the production values are decent, the scene is written skillfully enough, and Taaffe O'Connell plays it out quite well.
Evil Toons was created by Fred Olen Ray, master of exploitation flicks. It concerns an animated beast resembling the Tasmanian Devil who preys upon live-action co-eds. "First they undress you, then they possess you!" 'Nuff said. Madison Stone's scene - the only one of substance, given what we're talking about here - involves the beast ambushing her, stripping her, and, ostensibly, penetrating her. It's not my all-time favorite, but it is good, if only because it's unusual.
Two notes: Evidently, Madison Stone either was a hardcore porn star or became a hardcore porn star after she appeared in this movie. I haven't seen any porn with her in it, though. Also, the legend David Carradine appears in this movie - go, David! Fortunately, he isn't undressed or possessed.
Humanoids from the Deep - the 1980 version, not the mild remake - features a number of on-screen monster assaults, although only two go "all the way." The first is a little shorter and a little less graphic than the second, so I opted to include the second in the poll. In essence, a bunch of mutant fish men - damn your hubris, scientists! - lumber out of the ocean near some fishing village in the Pacific Northwest, and, naturally, feel obligated to molest beautiful women. Yeah, it's an old story, but it's done so well here that I felt like I had to include it. Lynn Theel is a typical big-boobied bubbleheaded blonde. She looks good enough to a shallow guy like me, and she manages to play the scene straight, which is admirable considering the laughable costuming.
I almost forget: Hail Corman!
Finally... Re-Animator. I just had to include something based on a Lovecraft story because I'm such a big fan of his. The movie's not actually that bad - I mean it's a bad movie, yeah, but it's a good bad movie, y'know? Barbara Crampton is great as always; like Ellen Sandweiss, she's laid back and possesses a good sense of humor, so she's perfect for something like this. The scene in question is arguably not Crampton's most monster-rape-related scene (that honor would probably go to her scene in From Beyond), but come on - a woman being molested by a villain's severed head? Gold, baby... pure gold.
(Oh, and that loud whirring sound? The one coming from the direction of Providence, RI? That's Lovecraft spinning in his grave.)