Adoption in Islam
There are several factors and points of clarification when it comes to how Islam handles the concept of adoption. Because of the strict segregation between genders, there are sensitivities at hand which the non-Muslim never has to consider, but alhamdulillah, it is still not only very doable, but also highly encouraged within the religion.
Inshallah, I hope to present the relevant proofs and evidences in this post.
Establishing a Mahrem Relationship
There are a few ways that mahram relationships are established—that is, a type of relationship whereby persons are forbidden in marriage to each other. The women forbidden to men are mentioned comprehensively in the Qur’an:
"Forbidden to you (for marriage) are: your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your foster mothers who gave you suck, your foster milk suckling sisters, your wives’ mothers, your stepdaughters under your guardianship, born of your wives to whom you have gone in—but there is no sin on you if you have not gone in them (to marry their daughters),—the wives of your sons who (spring) from your own loins, and two sisters in wedlock at the same time, except for what has already passed; verily, Allaah is Oft‑Forgiving, Most Merciful”
—Qur’an (al-Nisa’) 4:23
To break this down, we can see that the categories include:
Blood Ties
Your parents, siblings, maternal and paternal aunts and uncles, grandparents and all direct ascendents and descendents are default mahrams. This is a permanent state of being; you are never allowed to marry this category of people.
Marriage
Some relationships become forbidden to you once a marriage takes place, such as your parents-in-law. The father-in-law, for example, is someone who is mahram to you even if you divorce his son down the line. He becomes a permanent mahram by way of marriage.
Through Breastfeeding
The Prophet (ﷺ) said in a hadith that "what becomes mahram (forbidden for marriage) through breastfeeding is that which becomes mahram through ties of kinship."
—Sahih al-Bukhari 2502, Sahih Muslim 1447
As for the breastfeeding, then it is 5 full feedings which, when completed, renders the person a mahram to the woman.
On the authority of A’ishah (radhiAllah anha), who said, "Among the things that were revealed of the Qur’an was that ten definite breastfeedings make a person a mahram, then that was abrogated and replaced with five definite breastfeedings, and the Messenger of Allah (ﷺ) passed away when this was among the things that were recited of the Qur’an."
—Sahih Muslim 1452
Thus a woman may adopt a male child, breastfeed him 5 full feedings and he becomes mahram for her, bi’idhnillah.
Adoption
It is that last category of "mahramship" which is relevant for the topic at hand. As mentioned, an adult woman is able to adopt a male child and there is nothing wrong with that, provided the 5 full feedings are completed.
Age of the Child
One issue of dispute is the age of the child. The majority of scholars are agreed that as long as the feedings are done before the child reaches the age of 2 years old, the mahramship is established.
There is also a minority opinion, which was followed by A’ishah (radhiAllah anha) in which the age of the child doesn’t matter at all, as long as the 5 feedings are established. Allahu ‘alim on which is correct; the first is safer, but I personally also follow the ijtihad of our Mother A’ishah (radhiAllah anha), who was a scholar and mountain of knowledge in this religion.
Naming the Child
What is prohibited in adoption is to give the child your name or otherwise attribute him or her to yourself, such as giving them your last name or calling them "son of [yourself]" as is the typical Islamic naming convention ("Fulan ibn Fulan").
"nor has He made your adopted sons your real sons. That is but your saying with your mouths. But Allah says the truth, and He guides to the (Right) Way.
Call them (adopted sons) by (the names of) their fathers, that is more just with Allah. But if you know not their fathers (names, call them) your brothers in faith and Mawaleekum (your freed slaves). And there is no sin on you if you make a mistake therein, except in regard to what your hearts deliberately intend. And Allah is Ever Oft-Forgiving, Most Merciful"
—Qur’an (al-Ahzaab) 33:4-5
Your Husband and the Adopted Child
Going back to the establishing of the mahram relationship, a woman’s child established through breastfeeding takes on the same ruling as those of blood ties. A woman’s daughters become her husband’s mahrams if the marriage is consummated. If your husband has children already, then they also become mahrem for you.
So if you marry a man and the marriage contract is consummated, your children become mahram for him, alhamdulillah, and this applies for your children which you birthed, or those which you adopted and breastfed, as has been explained above.
Breastfeeding
For those who have never given birth (whether single or married) and thus are not naturally lactating, it’s possible to induce lactation via manual manipulation or pharmaceutical intervention, in sha’ Allah. These regiments normally take some time (a few months), so one should plan ahead.
Conclusion
There is nothing wrong with a person adopting children (whether male or female) and later getting married, in sha’ Allah. May Allah grant us the best of this life and the Hereafter. Ameen.