The paper is here. Brian Earp has written many other papers on the matter of infant genital cutting, so if you're ever looking for more information on the topic I'd recommend you become acquainted with him

The abstract:

Critics of non-therapeutic male and female childhood genital cutting claim that such cutting is harmful. It is therefore puzzling that 'circumcised' women and men do not typically regard themselves as having been harmed by the cutting, notwithstanding the loss of sensitive, prima facie valuable tissue. For female genital cutting (FGC), a commonly proposed solution to this puzzle is that women who had part(s) of their vulvae removed before sexual debut 'do not know what they're missing' and may 'justify' their genitally altered state by adopting false beliefs about the benefits of FGC, while simultaneously stigmatizing unmodified genitalia as unattractive or unclean. Might a similar phenomenon apply to neonatally circumcised men? In this survey of 999 US American men, we find the grater endorsement of false beliefs concerning circumcision and penile anatomy predicts grater satisfaction with being circumcised, while among genitally intact men, a trend in the opposite direction occurs: greater endorsement of false beliefs predicts less satisfaction with being genitally intact. These findings provide tentative support for the hypothesis that the lack of harm reported by many circumcised men, like the lack of harm reported by their female counterparts in societies that practice FGC, may be related to holding inaccurate beliefs concerning unaltered genitalia and the consequences of childhood genital modification.

It ultimately lends evidence to what many here have already hypothesized: circumcised men who are 'fine with it' don't know what they're missing and adjust to it, rather than that it's actually better to be circumcised

If a person is circumcised, there's not a whole like he can do about it now (except attempt regrowth). The point is of course not shaming men who are already circumcised; the point is that it's not something a person should force onto his child, ever. It is not better, it's not beneficial, it's not even just as good

An interesting thing to note is that the phenomenon is the same for women who have been circumcised as infants, who mostly also don't feel unsatisfied with it. That's why I don't understand when people argue "male circumcision is okay because men don't complain", since women don't usually complain either. But people don't say it's okay to circumcise infant females because adult circumcised women are satisfied with it

I understand why it's a controversial topic, because there are a lot of circumcised men who obviously don't want to see themselves as lacking something important, and even may feel okay with it themselves due to their ignorance about the issue, as this paper has shown

But it is important to recognize that in fact, sadly, there is something missing, and people can't keep doing it to their children with the justification "it happened to me, and I'm fine with it"