If her guardian refuses to give her in marriage to a man who is already married and has children, is he unjustly preventing her from getting married?

If this man is good in terms of
religious commitment and character, and you want to marry him, then tell him to
approach your guardian (wali), and tell your guardian
about his good qualities and that you accept him. If your father agrees to
accept him as a husband for you, then praise be to
Allah. If he refuses and gives an acceptable reason for that, then it is not
permissible for you to try to marry him through the imam of the mosque or
anyone else, because there is saheeh evidence that
the approval of the guardian is a condition of marriage being valid and that it
is not permissible to overlook him so long as he is not unjustly preventing
marriage. Unjust prevention of marriage means preventing a woman from marrying
a compatible man of whom she approves.

For example, the Prophet (blessings and
peace of Allah be upon him) said: “There is no marriage except with a
guardian.” Narrated by Abu Dawood, 2085; at-Tirmidhi, 1101; Ibn Maajah, 1881, from Abu Moosa al-Ash‘ari; classed as saheeh by al-Albaani in Saheeh at-Tirmidhi

And he (blessings and peace of Allah be
upon him) said: “Any woman who gets married without the permission of her
guardian, her marriage is invalid, her marriage is invalid, her
marriage is invalid.”

Narrated by Ahmad, 24417; Abu Dawood, 2083; at-Tirmidhi. 1102.
Classed as saheeh by al-Albaani
in Saheeh al-Jaami‘,
2709

If it is proven that marriage has been
unjustly prevented, guardianship passes to the next closest male relative (on
the father’s side), such as the (paternal) grandfather, then the brother,
brother’s son, paternal uncle and so on. If none of these relatives are present
or they refuse to give the woman in marriage, then she may be given in marriage
by the Muslim judge if there is one, otherwise the imam of the mosque or a
Muslim of similar standing may give her in marriage.

But if the guardian refuses to give a
girl in marriage to a married man who has children, this is not regarded as
unjustly preventing marriage, because he thinks that it is not appropriate for
her or he fears that she may suffer in this marriage, or he fears that there
may be problems between her and the first wife, as happens in many cases of
plural marriage, many of which end in failure and termination of married life.

The guardian has to look at what is in
the best interests of the female relative under his guardianship. In most cases
he is better able than she is to let reason and wisdom prevail over emotion,
hence Islam has given him this duty.

An-Nasaa’i
(3221) narrated that Buraydah said: Abu Bakr and ‘Umar both proposed
marriage to Faatimah, but the Messenger of Allah
(blessings and peace of Allah be upon him) said: “She is too young.” Then ‘Ali
proposed to her and he gave her in marriage to him. This hadeeth
was classed as saheeh by al-Albaani
in Saheeh an-Nasaa’i.

As-Sindi said
in his commentary on an-Nasaa’i: The words “Then ‘Ali
proposed to her” indicate that happened straight afterwards without any delay,
as is indicated by the word fa (translated
here as “then”). Thus it is known that the Prophet (blessings and peaces of Allah be upon him) thought that she was too young
for them (i.e., Abu Bakr and ‘Umar)
but that did not apply in ‘Ali’s case. This indicates that being of the same
age or close in age is something to be paid attention to because it is more
likely to lead to harmony. End quote.

Thus it is known that it is possible to
reject a man who is of good character and religiously committed, if the
guardian thinks that his daughter is not suitable for him in terms of age or if
he fears that she may suffer in the marriage, or that she may have troubles
with the co-wife, or he hopes that she may receive a proposal from someone who
is more suitable for her.

It says in Asna’l-Mataalib
(3/108): It says in al-Ihya’: just as it is mustahabb to marry a virgin, it is mustahabb
not to give one’s daughter in marriage except to a virgin who has never been
married, because people usually feel more at ease with the first spouse. End
quote.

So do not object to your parents if they
reject this suitor, because they have more insight into married life than you
and they are the keenest of people to get what is good for you. Their opinion
is based on experience and reasoning, not emotion, and perhaps Allah will bless
you with something good by virtue of your obeying your parents.

And Allah knows best.