Asking for a harsh register is fine

No, asking for SPH is not rude, provided the judge you are asking has said they offer it. The register itself is not the problem; skipping the checking is.

What actually makes an SPH request rude

Assuming every judge does this register without checking their stated boundaries first. Writing the request in the harsh tone you are asking for, rather than describing it plainly - the register belongs in the clip, not in your message. Pushing past a stated limit, or treating a decline as an opening to negotiate rather than a closed door.

None of those are about the register being humiliating. They would be equally rude for a request in any register, because they are about respecting a boundary, not about the content of the ask.

What a proper ask looks like

Reading a judge's stated boundaries before you ask is the first step, ask a one-line question if it is not clear whether they cover it, then brief plainly: the tone you want, roughly how far, and anything specifically off-limits. A judge who offers SPH has already decided this is work they choose to do, priced and boundaried the same as any other register they perform. Treating the request as ordinary business, because it is, gets you a better clip than treating it as something you need to justify. The wider interest is not rare either: in a representative Belgian sample of 1,027 adults, Holvoet and colleagues (2017) found 46.8% had ever performed at least one BDSM-related activity, and 12.5% did so on a regular basis.

Why the register is not the issue

A judge performing a harsh register is doing skilled work: sustaining a tone convincingly, on brief, for a stranger, which is not the same thing as an unfiltered opinion. The mechanics of briefing this register well - being clear, being courteous in the ask itself even while requesting harshness in the clip, and stating limits - are what separate a commission a judge is glad to take from one they decline on sight.

This is also a register with no real equivalent on the automated side. AI Penis generates from a template tuned for consistency, not a performed persona holding a tone across a whole clip, and Penis Rater returns a number with no register at all attached to it. If what you actually want is a plain measurement with no performance layered on top, Measure My Cock is the more direct route to that. Rate Cock lists which registers each judge takes for exactly this reason - check first, ask properly, and the register you want is a normal commission, not an imposition.