LOR Questions family planning

Question

One of the most common questions Rabbis tend to get is about contraception. Specifically a discussion about family planning, and how long one can use contraception for. Before the man has fulfilled his mitzvah of Peru Urvu to avoid doing a Mitzvah Dioressah could be problematic. However, once they have a boy and a girl there is now a mitzvah drbanan of lsheves.
My question is why do Rabbis say this is a question that must be asked, I don’t ask a Rabbi if I should have guests over for Shabbos. What is the difference they are both avoding doing a biblical commandment (one is Hachnosas Orchaim the other peru urvu). I don’t ask my Rabbi if there is a ‘heter’ to avoid visiting my parents for a year to avoid Kivud Av? Why by family planning is there such an emphasis on asking the Rabbi when other mitzvos (which are not time bound) there is no emphasis to ask?
The answer I am looking for should be based on sources why lsheves/pru urvu is more stringent than other Yoreh Deah cases.

EDIT

Based on the comments this has been added: Since birth control until more recently was highly problematic, from a Halachic standpoint, there was never any questions about using it, unless Pikuach Nefesh it was Assur. In the past few years where the actual BC is not an issue itself, there has been little public talk in terms of Halcahic writings on it. We are left to infer from the writings which we will call the source. This source can be the Rambam as answered by Dov. We don’t paskan by the Rambam if the SA does not concur with the ruling. We have a Gemora as RabbiKaii and Dov wrote. (Besides the Ramchal in his letters disagreeing with this concept) The Shulchan Aruch does not bring this as his opinion.
We are left with the Shulchan Aruch which says if someone remarries he should remarry to a woman who can have children even if has been Yotze peru urevu. We see from this that he is pro having more children if he can. The minhag has always been to ask questions like this to our Rabbis, I shall continue to do so.
I am still hoping someone will be able to help find a source that can help me understand the Halchic (NOT HASHKAFIK) reason why to ask whether one can take a break after being Yotze Pru urvu (+1) when timed mitzvos generally we don’t ask (for example I don’t know anyone who asked a rov how much money they should put away a month for writing or buying a sefer Torah)

Answer 1

Possibly because of the following:

Rambam in Hilchos Ishus 15:16:

אַף עַל פִּי שֶׁקִּיֵּם אָדָם מִצְוַת פְּרִיָּה וּרְבִיָּה הֲרֵי הוּא מְצֻוֶּה מִדִּבְרֵי סוֹפְרִים שֶׁלֹּא יִבָּטֵל מִלִּפְרוֹת וְלִרְבּוֹת כָּל זְמַן שֶׁיֵּשׁ בּוֹ כֹּחַ. שֶׁכָּל הַמּוֹסִיף נֶפֶשׁ אַחַת בְּיִשְׂרָאֵל כְּאִלּוּ בָּנָה עוֹלָם

Even once a person has fulfilled the Mitzvah of having children, it is a commandment from our Rabbis not to stop having children all the time one has the strength to do so, since for every additional soul in Israel it is like he has built a whole world.

Also note the Minchas Chinuch on the first mitzvah who says that the mitzvah of Peru Urevu differs from other commandments in that it is an ongoing mitzvah and does not have a time limit.

Finally, also refer to the Gemara in Yevamos 63b:

אָמַר רַבִּי אַסִּי: אֵין בֶּן דָּוִד בָּא עַד שֶׁיִּכְלוּ כׇּל הַנְּשָׁמוֹת שֶׁבַּגּוּף, שֶׁנֶּאֱמַר: ״כִּי רוּחַ מִלְּפָנַי יַעֲטוֹף וּנְשָׁמוֹת אֲנִי עָשִׂיתִי״. תַּנְיָא, רַבִּי אֱלִיעֶזֶר אוֹמֵר: כׇּל מִי שֶׁאֵין עוֹסֵק בִּפְרִיָּה וּרְבִיָּה — כְּאִילּוּ שׁוֹפֵךְ דָּמִים, שֶׁנֶּאֱמַר: ״שׁוֹפֵךְ דַּם הָאָדָם בָּאָדָם דָּמוֹ יִשָּׁפֵךְ״, וּכְתִיב בָּתְרֵיהּ: ״וְאַתֶּם פְּרוּ וּרְבוּ״.

Rabbi Asi said: The Messiah, son of David, will not come until all the souls of the body have been finished, i.e., until all souls that are destined to inhabit physical bodies will do so. As it is stated: “For the spirit that enwraps itself is from Me, and the souls that I have made” (Isaiah 57:16). It is taught in a baraita that Rabbi Eliezer says: Anyone who does not engage in the mitzva to be fruitful and multiply is considered as though he sheds blood, as it is stated: “Whoever sheds the blood of man, by man shall his blood be shed” (Genesis 9:6), and it is written immediately afterward: “And you, be fruitful and multiply” (Genesis 9:7). (Sefaria translation and notation)

Answer 2

There are a lot of meta-halachic issues at play. (See the question in general about "why ask a rabbi?".) But from a strict theory-of-halacha angle, one note I've heard is that "not inviting guests" is passive. Any form of birth control (other than total abstinence, I guess) involves taking some action. This could be considered bitul asei -- actively avoiding a "yes-do" mitzvah.

Answer 3

I think that there are multiple reasons for this phenomenon.

1. Most people have not learned the sugya of what types of birth control are permissible and in what circumstances. These topics are rarely talked about in public and aren't published in English seforim for the layperson.

2. The mitzvah of sheves and l'erev al tanech yodecha, makes it that there can be no blanket heter. There is a constant chiyuv.

3. Moreover, sheves/l'ereve is a mitzvah that is given great importance in halacha. The Gemara in Chagigah 2b uses sheves as a basis for forcing a master to sell a slave. And Tosafos on the spot says that sheves is an even more powerful mitzvah than pru ur'vu. One is allowed to sell a Sefer Torah just to be mekayem sheves/l'erev (see Shulchan Aruch Even HaEzer 1:2, 8 and the Bais Shmuel on the spot). Which does not apply to any other mitzvah other than learning Torah. On the other hand, the mitzvah is only d'rabanan so people are more likely to be mezalzel in it.

The combination of these 3 factors means that there is a great danger that if a shaila is not asked, people will make major mistakes and be oiver on hashchosas zera or bitul sheves. Moreover--as discussed by Rav Moshe in Even HaEzer 4:67--there is a danger of people learning heterim from their friends and applying it to themselves where they shouldn't.

Of course, if you have learned the sugyos yourself and are confident in your ability to pasken for yourself, that is great. You don't need to ask a shaila because you know the answer already.