Every man is obligated to marry a woman in order to be fruitful, and to multiply and anyone who doesn't engage in being fruitful and multiplying is as if he spills blood, and lessens the appearance, and causes the divine presence to depart from Israel. Rem"a: He who does not marry is not allowed to make a blessing or to engage in Torah etc. and he is not called a man, and when he marries a woman, his sins are cast into doubt, as it is said: "One who has found a wife has found goodness and obtains favor in the eyes of God." (Proverbs 18:22)
One can only sell a sefer Torah in order to learn Torah or marry a woman.
It is incumbent on every man that they should marry a woman at the age of 18 and the diligent get married at 13 and this mitzvah is for those who choose it, but before the age of 13 one should not marry, because it is similar to harlotry. If 20 years go by and he has not taken a wife and he who lets 20 years pass, or he who does not want to marry, the courts can force him to marry in order to fulfill the mitzvah of being fruitful and multiplying. What are the things that might delay marriage? If he is busy with Torah, or if it is too burdensome and he fears to marry a women in order that he is not burdened in his livelihood and he will not fail in Torah it is permitted to delay. Rem"a: In this time, its custom that one does not force in regards to this. Likewise he who does not fulfill the mitzvah of being fruitful and multiplying and he comes to marry a woman who is not a bat banim, for example...
He who spends all of his entire life engrossed in Torah study like Ben Azai and did not marry a woman is not considered a sinner, as his urges did not overcome him.
Once a man has had a son and a daughter, he has fulfilled the obligation to be fruitful and multiply. And a ben siris (castrated son) and a nekivah ayalonit (girl who doesn't achieve signs of puberty) do not count. Rem"a: The ram is the male of the flock, in other words, that the woman has a male nature and those signs she does not have like breasts of a woman and her voice is thick...doesn't stick out from her body like women]
If a man and a woman bear children and they die, and they leave behind sons, the man dies having fulfilled the mitzvah to be fruitful and multiply. What are the circumstances for this case? When there are grandchildren, a male and a female, and they came from the son and the daughter (of the obligated individal of halakha 5) despite the fact that the male grandchild comes from the daughter and the female grandchild comes from the son, since they came from his two kids, they fulfill the mitzvah of being fruitful and multiplying. But if he has a son and a daughter and they die and only one grandchild remains he still has not fulfilled his obligation. Rem"a: if the son is a mamzer or deaf-mute, or an idiot, or a minor, he has fulfilled the mitzvah.
If he had sons that in his lifetime and he was an idolater and all parties converts (to Judaism) it this case his mitzvah is fulfilled. But if he has children when he is a slave and he and they are freed he has not fulfilled his obligation until he bears one after he is freed.
Even though he fulfilled the obligation of be fruitful and multiply, he is forbidden from being without a wife and he needs to marry a woman who is capable of bearing children (see Yevamot 61b). He may sell a sefer Torah in order to do so if he has a doubt that he has enough children or if he has a doubt that he has married a women of child bearing capability. If he does not have children he sells so that he can marry a woman capable of bearing children but if he has children he should not sell it and marry a woman who is capable of bearing children, but he should not be without a wife. And some say, that even if he has sons, he sells the sefer Torah to marry a woman capable of bearing children. Isserless: However, if it is recognized that the will not be more grandchildren and it is not appropriate that more will not be born to him, he marries a women who is not capable of bearing children, and thus if he has many sons and he fears that if he marries a woman of child bearing age an altercation will come and discord will come between the children and his wife he is permitted to marry a woman who is not capable of bearing children but it is forbidden to settle down without a wife because of this fear.
A man may marry a number of women provided that he has the means to sustain them, and what we find is that the sages gave worthy suggestion that a man marry no more than four women so that he may fulfill his sexual obligation once a month. In a place where they are accustomed to only marry one woman they are not permitted to marry another woman. Note: it is forbidden to marry two women in two places.
RABBEINU GERSHOM DECREED CHEREM for one who marries [another] in addition to his wife, but in regards to a Leverite marriage, he did not decree cherem, and such is also the case with a betrothed woman. Rema: If he doesn't want to marry another, but rather to release her, and this is the rule in any place that a mitzvah would be delayed -- for instance, if he was married to his wife for ten years and she didn't give birth (Mordechai, Perek 'Hacholetz' Rashba siman 280 and Maharam Padua Siman 19). However there are those who disagree and say that Rabbeinu Gershom's decree applies even in a case in which a mitzvah is involved, and even in the case of a Levirite marriage, and one must do Chalitza (see Hagahot Mordechai in Yebamot and Ketubot). And in the case where the first wife is not easily divorceable, e.g. she became mentally infirmed, or the husband is legally allowed to divorce her but she won't accept a Get from him, one should be more lenient and allow him to marry another (see Rashb"a). And this case if she is betrothed and neither wants to marry him nor to be released from the betrothal. AND HIS [Rabbeinu Gershom's] DECREE was not unilaterally accepted in all the lands. Rema: Particularly in a plce where it is known that his decree wasn't unilaterally accepted, but basically it applies everywhere. See Section 14 Siman 228 for what one should do if he moved from a place where they are stringent on this ruling to a place where they are more lenient. AND HE ONLY EXTENDED THE DECREE until the end of the 5th centure (i.e. 500 years from when he said it). Rema: In any case, in all of these lands, the decree and the custom stand in their place, and we don't marry two women, and this is a punishable offense, and we force someone who did marry two wives to divorce one of them. And there are those who say that in this day and age, we shouldn't punish one who disobeys this decree because the fifth century [since it was declared] has already passed [and so it is expired], but this is not our custom. There are those who say that one whose wife refuses to accept a Get, he can serve it to her through someone else and then marry another, and this is the custom in some places. And in a place where this is not the custom, one should not be stringent, and a man may marry a second [wife] without divorcing the first.
It is good to make a Takanah in relation to forbidding and to excommunicate a person who takes more than one wife
If he swears that he will never marry another woman but 10 years go by and she has not born a child, this will be explained in Siman 118.
A woman is not obligated in the mitzvah of being fruitful and multiplying [this idea is talked about in siman 154] RMA: nonetheless some say that she should not remain unmarried, due to suspicion.
Laws associated with someone who marries a woman and 10 years pass and she does not bear a child will be explained there (siman 118).