Let me get this straight. You're supposed to be strong enough to
hold down a woman,
rip her jeans off,
pull your pants off,
get your dick hard instantly,
somehow stop her from kicking you in the dick,
hold her down and fuck her all while staying hard.
You seem to be assuming a lot of stuff here. For example, why do you think she is wearing jeans? You pick the stiffest hardest-to-get-off kind of legwear. Much easier to lift a skirt, or pull off a pair of sweatpants/yoga pants.
Dick-hardening would not need to be instant. A guy might already be hard, or might become hard in response to a woman's struggle.
If you look at feedback for certain manga for example, while some guys are turned off at the idea of violating women, the idea that a woman resists sex could be arousing in that it might speak for her having standards, maybe being purer, maybe even being a virgin or at least less promiscuous compared to other women who resist less.
Kicking someone at the dick is pretty hard to do in close-combat grappling range. GURPS 3e had the right idea for this when they assigned a -1 to damage penalty for kicks made at close range instead of 1 yard, and even then it was only people with the Karate skill who knew how to do it. For some reason GURPS 4e failed to acknowledge this, although I think the inference is that if you use kicking at close-range you are actually doing a knee (with thigh, not foot) but you have the same chances of falling as a general Kick Attack, as opposed to "Knee Strike" which lacks that chance of falling down upon a missed roll.
The simple matter of fact is, if I imagine how I would go about raping if that is something I'd choose to do (so far, it isn't, I choose consent or celibacy) my knowledge of grappling is enough that I could figure out how to do it and accomodate what you think are impassible barriers.
Basically, if you watch some MMA, you can figure out from the "guard" position that once you get in between an opponent's knees, they aren't able to kick/knee you. So obviously you would grapple the legs and pull them apart and get yourself in that position before releasing the legs, at which point you are immune to kicks unless they buck you off (and most women don't work their glutes enough to have the power to bridge that powerfully, they should be watching some Bret Contreras videos on how to do hip thrusts)
Once in that position and hands are freed from the need to grapple legs, you would only need one hand to remove your belt and undo a snap-button (trickier if it's a non-snap button, though still feasible with practise) and unzip. Similar for hers (which is why women wanting to make it harder to remove their pants should use non-snap buttons which most people require 2 hands to undo easily)
Since you can do it 1-handed, your other hand could be used to pin her arms. Even if your hand isn't big enough to wrap around both her wrists simultaneously, you could just grip 1 finger from each hand together, basically a fingerlock to prevent the hands from moving apart as you held them overhead.
Furthermore, if you put someone's hands beneath their torso and then just lay upon their torso, the combined weight of both torsos would be pinning the hands to the ground even if you were not directly grappling them at all. In terms of GURPS Technical Grappling this is called "Pass Limb". This would allow a rapist to free BOTH hands for undoing clothing clasps, with their victims' hands reliably being pinned.
All of this analysis shows it is definitely possibly for a resisting victim to be pinned and still progressively undressed, so I don't like your implication OP that rape somehow can't happen because of technical impossibility, it definitely can.
Another major consideration here, it's that it's hard to pull down underwear when the knees are apart, so they'll probably just pulled aside or torn/cut.
If they are pulled down, it will probably hold the knees together (preventing a rapist from getting between them to avoid kicks/knees) so to access the vagina or anus, a rapist would probably just lift the legs and enter from below, or turn them over and enter from behind.
Obviously you can't do knees in this position, and the ability to kick at full power is compromised. You're basically trying to kick like this:
You're basically just relying on 100% knee flexors (hamstrings, gasotrocnemius) to try and kick someone with the heel. It's not as much power as when you can do a thrusting movement combining knee extensors (quads) and hip extensors (glutes). Plus it's not very acccurate, and it's incredibly easy to parry. It's also not going to go anywhere near the groin of the rapist.
The major problem with the "underwear around the knees" thing is the rapist is basically positioning himself too far away from the arms of a victim for hands to be used as weapons against them. The legs (which they can maintain a grapple on to pull themselves into their victim) are basically improvised shields and serve as cover, plus it's very difficult to reach the hands below the hip unless you have the abdominal strength to create torso flexion and shorten the distance between the shoulder joints (cervico-thoracic vertebrae) and the pelvis (lumbar-sacral vertabrae)
This would be especially hard if the legs are pulled into the air and the pelvis is lifted like this piledriver position
vndb.org
Even if someone had the strength to crunch their upper body off the ground towards the hips (to help reach an attacker) the attacker could simply drop them onto the ground to injure them, or wait until they were tired out. The need to reach around the legs basically really makes it hard to attack when grappled like this.
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One major point you are missing here though is that fear is paralyzing, and paralysis/fear is not any indication of giving consent. Even if a rapist is unarmed, he has a fist that could shatter her face (or easily kill her with a punch, as you admit below) and that is a terrifying prospect to anyone, male or female. He doesn't need to physically grapple her at all, just terrify her into submission with threat of harm.
That is a completely understandable and forgiveable (well, there's nothing to forgive, they're not doing anything wrong) state to be in, and I'm not going to blame people in that state of paralysis (I too have been paralyzed by fear, I can empathize) or imply that they are somehow willing/cooperative because they freeze up and don't pull out some spontaneous Gracie-tier ground game and arm-bar someone trying to rape them.
TBH if some guy just stated grappling me out of nowhere, I might try and turtle up (cover my face) for a few seconds and not be able to process WTF is going on to be able to do countermeasures like knee a guy in the nuts. If I'm covering my face, I probably won't see him setting up a leg grapple and wouldn't be able to tense my hip adductor muscles to prevent my knees being pulled apart.
Even if I did get my head together and fought back before getting poopered, that's probably because as a guy I'm just conditioned into thinking about combat more. Women aren't encouraged to do that. Also, I'm probably more confident about my ability to survive a punch than a woman, since men naturally have thicker jawbones/skulls and shit.
I'd still probably be petrified if a guy had a weapon like a knife or a gun though. Even though intellectually I might think "there's a high likelihood he would miss and then I could disarm him" the off chance he would hit me and I would die could very likely paralyze me in fear for my life. This is something we might be overconfident about, but not truly know except when put in a situation like that.
All the grappling technicalities in the world I've explored about won't mean shit if our head isn't in the game, and it's not just people who put up a fight that deserve our empathy when they are sexually assaulted.
I'm guessing IT will take my exploration above as some kind of "how-to" guide, which is obviously BS, as it's common sense stuff anybody would probably think up on their own before attacking others. Spelling it out is important for defending against posts like the OP which under-represent the versatility of grappling to make it seem like victims of grappling are somehow being co-operative in their own assault.
And if a rapist hit someone in the head until they are unconsciousness, that usually KILLS them, unlike the movies.
While I agree with you here, it isn't necessary to go that far to subjugate someone. For example fear/intimidation alone can do it.
But experienced porn stars all need viagra or ED drugs to stay hard on film for a decent attractive willing woman?
Does anyone else see how obviously stupid this whole "violent rape" bullshit claim is?
Even if they are attractive visually speaking, knowing she's probably had hundreds of cocks in her is something the male performer can't tune out, so I wouldn't blame him for getting soft.
You also have to keep in mind here that these guys are probably having to constantly stop and look at a hairy/obese director telling them what to do, which is probably frustrating.
