We don't know because no-one has been allowed to test the male secretions. The male endocrine system is different to the female, the particular male did not give birth and therefore the secretions cannot be tailored to the child. Mammary glands will not have matured at puberty, etc, etc. From the little we know, amounts are tiny as well. So my guess is that it is nothing like mother's milk and far inferior to formula, which is at least nutritious and known to be safe, if not bespoke.
A woman who is given drugs to aid lactation is taking versions of natural female hormones and only has to use them to start her off. Whether the baby is hers or not, everything is in place because she is female. Like the non-birthing mum of the lesbian couple you mention.
The liquid secreted from a man's nipples exposes the child to unquantified risks. This does not benefit the child.
A man who wants to feed his child can cradle it in his arms and feed it with a bottle of formula or his female partner's expressed milk.
The difference between that man and the male who wants to breastfeed is that the second man wants a child to suck on his nipples. I don't see how that benefits the child, either.