There are several possible reasons why other men might be upset, although your own equipment still works perfectly normally:

• Just like women's sexual responses differ, men's sexual responses may differ, as well. I've learned from a friend, who's had many male partners, that some men get intense pleasure from manipulation of their foreskin. Some can even reach orgasm that way. I've learned from several (intact) men on Reddit and Lemmy that their primary source of sexual sensation is their foreskin, rather than their glans. Losing a major source of pleasure could be upsetting.

• This same friend also reports that, in his experience, intact men have better awareness of their own state of arousal, and better control of it. In brief, they can "last longer." This is anecdotal, of course, but I seem to recall reading some research to back that up. That's part of the reason why he's upset by his being circumcised.

• "Circumcision" is not just one thing. It ranges from the traditional bris (a small snip at the tip of the penis, so that the tip of the glans just peeks out) to amputation of the entire mobile skin system of the penis (about 15 sq. in. of adult tissue gone). I would imagine that men who have drum-tight skin on their penises, and must use lube to facilitate penetration or masturbation, might not like it, whereas a man whose glans was still covered when his penis was flaccid might not notice much difference.

• The dorsal nerve of the penis can be severed during the procedure, removing sensation from the glans almost entirely, leading to erectile and performance issues, as well as greatly reduced enjoyment of sex.

• The healing of the circumcision wound can go not-quite-perfectly, leading to adhesions, assymetry, tight frenulums, phantom pain, and scarring. Journalist Gary Shteyngart wrote an essay about the odyssey of pain that he was thrown into when a skin bridge (an adhesion) on his penis became infected. Worse, I recall a letter published in Savage Love from a man whose circumcision scar was so thick and inelastic that it caused the end of his penis to go ischemic, then necrotic, and then fall off when he was an infant. He's left with a stub of a penis, and a pretty good reason to be upset about circumcision, I'd say.

ETA: I did not think of this, but @theleadensea@sh.itjust.works pointed out that removal of the foreskin complicates bottom surgery for trans women, I would guess because it gives the surgeon less tissue to work with.