
I think his reasoning was also for himself. I think ML wanted to see how he felt about this person as a person. Disclosing the info would have caused MP to likely be distressed and feel betrayed by his parents. Then, he would fight against getting to know ML, or feel pressured to act a certain way with ML. ML knew that, or suspected it, and wanted to see for himself, if they actually meshed as just two individuals without the chosen one nonsense on top of it all. The fact that he never registered to find his namer pair, shows that he also didn't want to be tied to "fate".
Also, deception isn't always a negative. If the truth does more harm to an individual (ie if their hair looks bad to you but they either love it or are too overwhelmed to fix it) then deception can be a good thing.
I think it is all very situational. We should also keep in mind the ages of the individuals in the story. The average man's frontal lobe, which is in charge of decision making, isn't fully formed until 28. It's actually one of the reasons men are more likely to die or get into accidents during their 20s when compared with women.

Yes, I agree. The angle I was taking was more so answering those confused as to how this was considered “bad”, “lying” or “manipulation.” It’s a different conversation entirely on the reasoning behind it and whether there’s any percentage of justification, which (depending who you ask) there’s always a way to support your choice.
An extreme example that totally has nothing to do with this but I am ONLY using for context of justification: “Mr. Scientist, why are you killing this entire population of one eyed frogs?” “Well, because then only the two eyed frogs can produce which will one day let ALL frogs see better. It’s for the greater good, so of course I’m justified enacting this deplorable action!”
Obviously, this is a different circumstance, but it goes to show that in problematic relationships/situations (not this yet) where deception and/or manipulation become a habit, justification is a tricky line to walk! If he really wanted to see how they got along without the complication of their “fate” he could have skipped having him strip and then seeing if they meshed, dated naturally and then determined later if he had the name when sex came up organically. But he didn’t unfortunately, which will lead to issues down the line no doubt.
You have good points though!!
The reason it’s not great ML isn’t disclosing that MC is actually a namer (specifically his) is because now that we know they are going to develop feelings for each other/what is most likely a positive relationship, once it is revealed (I’m guessing by MCs parents in some pseudo humorous way like “you’ve always had this birthmark”) MC is going to immediately overthink it as “ I never had free will and we never had feelings for each other, we were only driven together by the words on our skin just like all these posts on Reddit said, just like all these media outlets said, I can’t trust my feelings and now I certainly can’t trust YOU.”
That’s why it’s bad.
Also, please note, withholding information IS considered deception. Deception can be a form of manipulation, however manipulation is not necessarily deception. “All squares are rectangles”, you know? If he was actively manipulating, this would be gaslighting MC into thinking he wasn’t a namer. However, since he’s doing it indirectly, I’d say it’s just on the side of deception.