And that Joseph Smith's "marriages" were actually just sealings. So how do I prove this statement wrong with pro-lds sources ??

Read Doctrine and Covenants #132. It explains that the New and Everlasting Covenant of marriage or celestial marriage is plurality of wives.

They did a lot of sealings and lived polygamy even though it was illegal when it began in Illinois.


http://www.lds.org/scriptures/dc-testament/dc/132?lang=eng

This is a Seminary Lesson on the subject:

http://seminary.lds.org/manuals/doctrine-and-covenants-and-church-history-seminary-teacher-resource-manual/dc-trm-6-124-38-dc132.asp


http://www.wivesofjosephsmith.org/
CHeck out wives of Joseph Smith.

What was the purpose for the sealing without marriage, couldn't those women been both married and sealed to someone who didn't yet have either.

Why exactly did so many prominent members of the leadership have such a problem with it if they were just sealings?

I would also call on the missionary to provide documentation, it is their responsibility to back up their crazy claims.

You might ask the missionaries to define what they mean. I believe that generally speaking, "sealing" is intended to refer to a ceremony with the added "eternal" significance that usually takes place in the temple. But "sealing" was never intended to mean "sexless marriage" as some LDS would want.

Quinn has some good examples in his examination of post-manifesto marriages with the brief allowance for sealings to take place outside of temples to accomodate the concealment of plural marriages in Mexico: http://www.scribd.com/doc/62597201/Authorized-Post-Manifesto-Polygamy-by-Michael-Quinn

Also, if your friends persist that Smith's marriages were sexless, then you need to tell them he was sinning. While the BoM soundly condemns polygamy, its escape clause the LDS love to cite in Jacob 2:30 allows polygamy only for raising up seed: " For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things."

And D&C 132:63 says the same thing. Plural wives are "are given unto him to multiply and replenish the earth." So if Joseph was taking wives without the express purpose of procreation, he was violating his god's laws.