MIRROR: Download from MEGA
Generally speaking, there's no way to force a man to maintain a perpetual erection without arousal. Some drugs like Viagra can occasionally have a side effect called priapism, which is essentially a hard-on that last literally for hours. It's an extremely painful condition and can in some cases be permanently damaging to the cock. But there's no way, so far as I know, to intentionally induce the condition.
It's not that difficult to keep a cock hard. Long-term chastity without a chastity device allows the cock to get hard spontaneously, so that the sub gets hard very easily and stays hard for a long time. My slave is currently dealing with this because he's not allowed to masturbate. But this uses horniness as a way to induce an erection.
So I think you need to accept that the sub will be horny and get erections easily as a result.
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