CalMeacham is right. You have what’s called Mormonism, and you have what’s called The Church of Jesus Christ of Latter-day Saints: the first is a cultural phenomenon, and the second is a religion. There are many Latter-day Saints who live a cultural Mormon life, and there are many Latter-day Saints who don’t. Saints outside of predominantly Latter-day Saint areas (areas of Utah, Idaho, Utah, Arizona, etc.) tend to be less homogenous or “Mormon”. This is especially true for Latter-day Saints outside the United States and Canada.
There are many reasons why Latter-day Saints have large families. The Church disourages people not having children for selfish reasons - however, medical and other reasons are acceptable. Sure, people might be harrassed for not having kids, but that’s a manifestation of the cultural and not religious or doctrinal side of the membership. (The Restored Gospel is perfect, the members are not.)
Unlike most other conservative Christian denominations, The Church of Jesus Christ of Latter-day Saints is not adamantly opposed to birth control. It’s a matter between the couple and God, and must be prayerfully considered.
And almost all religions focus on marriage with members of the same religion and sect. Heck, in Islam, a Sunni marrying a Shi’a is huge issue and strongly frowned upon by both sides. (An uncle was introduced to a wonderful woman who was a potential wife for his son. His first question was, “But, isn’t she a Shi’a?” He would not tolerate his son marrying a Shi’a, despite my cousin’s not caring whether she was Shi’a or Sunni.) So, this isn’t specifically a Latter-day Saint hang-up.
Like most religious communities, this advocacy rests in an interest in the propagation and survival of the community. This might also be one reason why Latter-day Saints feel big families are good.
(Aside: Orthodox Jewish families can also tend to be big. Having children is a mitzvah, one of the first given to Adam and Eve. Latter-day Saints believe the first commandment given to Adam and Eve was “be fruitful and multiply”, which may be another reason why big families are good.)
And the reason Latter-day Saints are discouraged from dating non-Latter-day Saints is because dating is done with marriage in mind. Why date someone you can’t marry?
Latter-day Saints desire to “baptize” all the dead, regardless of creed, race, or morality. (Baptizing the dead - living people acting as a proxy for dead people, with the proxy being baptized on the dead person’s behalf. The deceased is free to accept or reject the ordinance). If they are eligible, the ordinance is valid and is acceptable. If is not eligible, the ordinance is null and void, even if the deceased wants it. There are those who believe shedding innocent blood makes all ordinances null and void for the living or the dead. But that’s just speculation. In this way, a punishment can be having an ordinance done for a person but then not having it be acceptable.
In any case, like all religions, Latter-day Saints believe that their religion is the supreme truth (although elements of the truth exist in all other religions), and so the work should be done for all. In deference to the Jewish community (Latter-day Saints and Jews get along quite well, this issue and missionary work notwithstanding), Holocaust victims cannot be baptized unless the person submitting the name is related to the victim. This standard exists for all names - one must be related to the deceased or get permission from the deceased’s relatives.
Oy, this should be enough for now. 
WRS