SPH is on the verbal humiliation fetish spectrum, obviously. It could be emotional masochism.
A heterooriented article has some men explaining it, but enforced chastity/ forced feminization is in there,too.
What You Learn About Masculinity from Making Fun of Men's Small Dicks for a Living
Dan Savage dealt with the inverse, the larger guy wanting to humiliate the smaller guy who was just fine with his size, TYVM. (From a November, 2017 column)
I am a pretty handsome gay and I am dating a gorgeous man. I am bottom only, and he is top only — so it's a good match. He seems sincerely interested in me and we are talking about being together. But here is the thing: He noticed that I have a rather small penis. I am under the average, and his dick is quite big and long. Since he discovered this, he fancies about "humiliating" me about my "small pee-pee." He would even like me to show it to his friends. I am not ashamed of the size of my penis because it's how I am made and I can't change it. But I wonder what this idea means for him. Is it a common turn-on for some top guys to imagine that their partner is smaller than them? Does it hide something else maybe?
— Humiliated Over Tackle (HOT in response)
Small penis humiliation (SPH) is a kink popular enough to have spawned a porn genre. There are more than 76,000 SPH-themed porn videos on XTube. Small penis humiliation is definitely a thing. And it can be a very good thing for guys whose erotic imaginations transformed their anxieties about having small dicks into a kink they enjoy.
But you are not one of those guys. You like your dick fine, and you've got the exact right attitude about your dick.
It's usually the guy with the small dick who initiates small penis humiliation games with his partner, HOT, not the boyfriend with the bigger dick.
But the issue here isn't stumbling over a rare small penis humiliation top in the wild, HOT, but whether or not you're into it. Are you into power play? Do you like being degraded? Does the thought of this dude ordering you to show your dick to his friends turn you on in any way? If the answer is no, no, and FUCK NO, then tell your potential new boyfriend to stop making fun of your cock. If the answer is maybe, maybe, and maybe under the right circumstances, then talk it over with him and work out when, where, and how you're willing to indulge his SPH kink.
It's worrisome that this guy didn't bother with obtaining your consent in advance, HOT, and if he doesn't recognize that he made a mistake and swear not to make a similar mistake in the future, well, then you'll have to DTMFA.