The initial erection process, in response to arousal, does not require direct penile stimulation. You know this already as otherwise most men wouldn’t have boners in their pants ! However, for the erection to remain, direct penile stimulation is required. This is in fact a good idea, since an erection can be really become a nuisance. For instance, running with an erection will only be fun for a few metres ! And this is only one example. (Never forget that our great ancestors didn’t have clothes to hold all this.) As a result, there has to be safeguards that will allow an erection to end the moment it isn’t required. The absence of direct stimulation is one of them.
This explains why the bottom will often completely loose his erection if his penis is left untouched, even though the bottom may enjoy a lot being fucked, even though he may find it very sexually pleasing.
Of course, as you see below, if erection is maintained, you may even cum ! But that is the exception rather than the norm.
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