Even though being fucked is truly enjoyable, the anal stimulation is not, in and by itself, erection-inducing. Which does not mean that it isn’t great. The feeling is unfocused, somewhat undefined and, more importantly I guess, does not originate from the penis. So even though the bottom may enjoy a lot being fucked, the bottom will often completely loose his erection. (Which means, of course, that he will not ejaculate…)
… Unless, of course, his erection is taken care of. If it is, the prostate massage and the well being felt as a result of being fucked may indeed be enough for the bottom to cum.
As we’ve seen, ejaculation is triggered by the brain when a certain level of arousal is reached. What causes this required level of arousal to be reached is usually an adequate stimulation of the penis, but even though the penis is the most efficient tool available to increase the brain’s arousal, the penis has by no means an exclusive ability. Knowing this, you’ll understand that absolutely any stimulation method can be used to cause ejaculation, provided this stimulation increases arousal enough to reach the trigger point.
This of course explains nocturnal emissions, but this also explains why anal stimulation can cause a man to cum.
It also helps that fucking entails the required passage to-and-fro against the prostate. As a result, the fucked man has a p-spot massage, which also contributes to his well-being and arousal. But the p-spot massage by itself is not enough to cause an ejaculation : the fucked man must be aroused enough to reach the trigger point.
All this to say that, for some men, the additive effect of mental imagery associated to the event, aural, anal and p-spot stimulation can be enough to arouse them beyond the point of no-return, and thus lead to an ejaculation. For most however, even though the experience is arousing and pleasurable, it won’t be enough to make them cum.
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