Firstly:

Plural marriage is Sunnah for the one who can afford it, and is not obligatory, according to scholarly consensus. See al-Mughni, 9/340.

Shaykh Ibn Baaz (may Allaah have mercy on him) was asked: Is plural marriage permissible in Islam or is it Sunnah?

He replied: Plural marriage is Sunnah for the one who can afford it. Allaah says (interpretation of the meaning):

"And if you fear that you shall not be able to deal justly with the orphan girls then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice"

[al-Nisa' 4:3]

And it is Sunnah because the Prophet (Peace And Blessings Of Allaah Be Upon Him) did it. He had nine wives and Allaah benefited the ummah through them. This is one of the things that applied only to him (Peace And Blessings Of Allaah Be Upon Him). Other men are not allowed to have more than four wives.

There are many benefits in plural marriage for men and women and for the Muslim ummah as a whole. Plural marriage enables everyone to lower their gaze and guard their chastity, to have many children, and for a man to take care of many women and protect them from the causes of evil and deviation.

As for the one who cannot afford that and fears that he will not be able to treat co-wives justly, he should settle for just one wife, because Allaah says: "but if you fear that you shall not be able to deal justly (with them), then only one". From Fataawa Islamiyyah, 3/202

Secondly:

It should be noted that in principle, marriage may be obligatory or mustahabb, depending on each person's situation and his need for marriage. Ibn Qudaamah (may Allaah have mercy on him) said:

With regard to marriage, people are of three categories:

1 – Those who fear that they may commit haraam actions if they do not get married. In this case marriage is obligatory according to most of the fuqaha', because a person has to keep himself chaste and avoid haraam things.

2 – Those for whom it is mustahabb, who are those who feel desire but they are certain that they are not going to do haraam things. For such people marriage is better than devoting oneself to naafil acts of worship, because Allaah and His Messenger (Peace And Blessings Of Allaah Be Upon Him) enjoined and encouraged marriage, and because the Prophet (Peace And Blessings Of Allaah Be Upon Him) got married, as did his companions, and the Prophet (Peace And Blessings Of Allaah Be Upon Him) and his companions only did that which is best. And because the interests served by marriage are many, because it protects one's religious commitment, and it protects the woman, keeps her chaste and takes care of her, and produces children, and increases the numbers of the ummah, and will make the Prophet (Peace And Blessings Of Allaah Be Upon Him) proud [of the numbers of his ummah in the Hereafter], and serves other interests, any one of which outweighs the benefits of doing naafil acts of worship, so it is better.

3 – Those who have no desire, either because they were created without desire or because they were created with desire but have now lost it, because of old age or sickness, etc. These people are of two types:

(a) Those for whom marriage is mustahabb, because of the general meaning of the evidence which enjoins marriage.

(b) Those for whom devoting themselves to worship is better, because they cannot fulfil the purposes of marriage and they would not be able to keep a wife chaste, which will cause her harm and this means that he will be keeping her without looking after her properly, and he will be taking on obligations and duties that he may not be able to fulfil.

The evidence which indicates that marriage is encouraged should be understood as referring to those who have desire.

And Allaah knows best.