Even though this is not in the Mishna Brurah, since the "Olam" is "Tumiling" in it, I'll go through the Sugyos

SEIF 1
WHEN ARE WE OBLIGATED TO FULFILL THE MITZVAH

The Rambam says that one must have relations for every Onah (time he's obligated to have relations with his wife) until he has his boy and girl. The Magid Mishna points out, even if his wife gives him permission (and that's why he wouldn't need to do it on account that he owes it to his wife) still he needs to do it on account of the Mitzva of P'ru U'rivu. It infers that after those children he may stop having relations with his wife's consent

The Bais Shmuel asks where does the Rambam get this from? Even though after the two children there is no Mitzvah of P'ru Ur'vu, still there is a Mitzvah to have children afterwards. Even though it's only MiDarabanan, still, what gives someone the right to forgo it?

The Taz infers from the Rambam that allows a man to marry a woman that has difficulty getting pregnant if he has a second wife which he can do that with (or if he already had children and fulfilled the Mitzvah.) But by his wife consenting not to have relations, he doesn't say that he's allowed to if he has a second wife. Only if he already had children he's allows to.

Therefore, the Taz concludes, that the Rambam is only going on those that have two wives, then before he's Yoitze P'ru Ur'vu he's obligated to try to have kids at any possible time that comes up. So even if there is no Mitzva to marry two wives, once you have two wives, you need to try at every Onah (even though his amount is double other people's.) But if he had children, then he doesn't need to do it for every possible time, as long as he has one wife that he can fulfill that Mitzvah with, (which in the first place doesn't need anymore) he doesn't need to do it with every wife he has and may refrain with his wife's consent.

According to the Taz, by us that don't take more than one wife, this Halacha is not practical. But the Chida says that the Rambam is talking about one wife. For the Mitzvah of P'ru U'r'vu he can't even skip one time until he fulfills the Mitzvah. But afterwards, even though there is still a Mitzvah to have more children, it's not a set amount how much and how often. Therefore as long as they have relations on a regular bases, they can skip some Onahs with her consent. The Orech Hashulchon Paskins like the Chida.