If you’re asking for strict double blind randomized trials then no of course not that research would be totally unethical. On the other hand we have a growing mountain of evidence that boys without fathers have comparably worse outcomes and so far as I know there’s no serious dispute.

Also, frankly, the suggestion that fathers are optional or even unnecessary is quite sexist. Just because we’re generally inured to anti-male sexism doesn’t keep it from being objectively wrong.