Further to my post above, let me say that I don't think asphyxiation has anything to do with sexual stimulation on the cross. After much study, I've come to the conclusion that although the physiology of crucifixion produces breathing difficulty, it doesn't produce hypoxia, i.e. greatly lowered blood oxygen levels. But the sensation of not being able to get enough breath does produce something called "dyspnea," which is more of a psychological panic. It's the kind of thing you'd experience if someone was holding you underwater, and your panic at being physically restrained from breathing, even for a few seconds, would lead you to struggle automatically to reach the surface. But when you are able to breathe again, you're ok very quickly. This is what I think crucified victims experienced once they had exhausted themselves and were forced to hang for longer periods. I think that their suffering followed a general progression as follows:
1. An initial frantic struggle driven by their pain, gradually slowing as fatigue became a factor.
2. A period of a few hours where they would hang for longer periods of time as they became more fatigued, raising themselves as pain in their wrists and upper body became unbearable. They would begin to experience breathing difficulties, or dyspnea, as they hung for longer periods.
3. A long period, up to several days, where their struggles were driven by a combination of agony and shortness of breath, or dyspnea. Dyspnea would drive them to take short, rapid breaths while hanging, and this would lead to hyperventilation, i.e. too much oxygen in the blood, and fainting. Once they fainted, their breathing would return to a normal state that was adequate to maintain their blood oxygen level, and they would return to consciousness within minutes. This is one of the real horrors of crucifixion - a victim could not escape the full extent of their punishment through fainting, hanging until they lost consciousness, etc. It would last until the final drop of their strength ran out or the executioner decided to end it. This point is very important, because it meant that the Romans could further show their power by keeping the victim alive and suffering until they made the choice to end it. It also may explain the shock expressed by the Roman guard at Jesus crucifixion, when he suddenly exclaimed "It is finished!" and then died; no victim ever announced that he was going to die now and just did it like that. I think that the writers of the gospels viewed that as a miracle in itself.
4. A final period where the victim's body was so drained of energy that even the involuntary breathing muscles no longer had the strength to work against the resistance created by his hanging position. Very gradually, over a period of perhaps a few hours, his breathing would become shallower until it reached a threshhold where it would no longer support life, and only then would he die. The body ordinarily keeps the involuntary muscles working to support life until it runs completely out of energy, so even after the victim was too exhausted to raise himself again, he might last for hours. I think the same thing is true when the victim's legs had been broken - although he could not raise himself, he would be forced to hang, for perhaps a few more hours, until his breathing muscles were too exhausted to work against the resistance. Any way you look at it, crucifixion was a horrible way to die.
Going back to the idea of sexual stimulation through asphyxiation, since the victim probably was not actually asphyxiating until the final stages of his suffering, I don't think he would experience any sexual stimulation from that. But it could certainly come from other sources, depending on his reaction to the feeling of nudity and exposure, direct pressure from a sedile or cornu, endorphins from the pain, etc.
Jedakk