The strongest answer is that this episode did not abolish care for orphans, nor did it create a private sexual license. It publicly ended a false legal fiction - that an adopted son is the same as a biological son - and the Prophet (PBUH) bore the social cost of demonstrating it.
Why the criticism feels forceful
The critic is not making a silly point. Quran 33:37 names a real marriage involving the Prophet (PBUH), Zaynab bint Jahsh, and Zayd ibn Harithah, who had been known as the Prophet's adopted son. The verse also gives a legal reason: to remove discomfort about marrying the former wives of adopted sons.
So the suspicion is this: a desire arises, then a revelation arrives, and the revelation both permits the Prophet's own marriage and changes a social rule. If someone already assumes revelation is human invention, the timing can look convenient.
What the verse actually says
And when you said to the one upon whom Allah had bestowed favor and upon whom you had bestowed favor, "Keep your wife and fear Allah," while you concealed within yourself that which Allah was to disclose, and you feared the people, while Allah has more right that you fear Him. So when Zayd had no further need of her, We married her to you so that there would be no discomfort for the believers concerning the wives of their adopted sons when they have no further need of them. And the command of Allah is ever accomplished.
Quran 33:37
The verse is striking because it is not written like propaganda. It openly says the Prophet (PBUH) feared people's reaction. If he were inventing a flattering text, this is a strange way to do it. The Quran records his hesitation and corrects him.
Classical scholars explained that the Prophet (PBUH) had been informed that this marriage would occur, but he kept advising Zayd to remain with Zaynab because he feared the public scandal. The Quran then commanded what social custom made difficult.
Adoption was not abolished - false lineage was
Islam did not abolish taking in children, loving them, feeding them, educating them, or treating them with mercy. It abolished changing their lineage and making legal rulings depend on a fiction. Just before this passage, the Quran says adopted sons are not literally one's sons and commands that they be called by their real fathers where known.
This matters. Lineage affects inheritance, marriage barriers, guardianship, identity, and family rights. Islam preserves the orphan's care without erasing his or her origin. A fostered or adopted child may be beloved, but the law does not pretend biology has changed.
Zayd himself was not hidden or degraded by this ruling. He is the only Companion named in the Quran. The point was legal clarity, not cruelty.
Why the marriage was a public burden, not an easy indulgence
If the Prophet (PBUH) wanted a self-serving arrangement, this was an unusually costly one. Zaynab was his cousin; she was not inaccessible to him before her marriage to Zayd. In fact, the Prophet (PBUH) had encouraged her marriage to Zayd, challenging Arab pride over class and former slavery. Later, after that marriage failed, the new ruling challenged another social taboo.
The marriage therefore served two reforms: first, that nobility is not based on tribal status; second, that adopted sonship does not create the same marriage prohibitions as blood sonship. Both reforms cut against Arab custom. Neither was a mere private indulgence.
What does this establish? It does not, by itself, force a skeptic to believe the Quran is from Allah. A person can always reinterpret any event through suspicion. But it does show that the charge of "convenience" is not the only serious reading, and not the best one. The text is morally and legally coherent: it preserves care, protects lineage, and requires the Prophet (PBUH) himself to carry the burden of a difficult reform.
The episode is uncomfortable only if adoption is assumed to mean legal replacement of birth lineage. Islam honors the vulnerable child, but it refuses to build family law on a fiction. That is the heart of the matter.