The following is from a letter Joseph F. Smith wrote to his sister Martha Ann, dated 15 May 1875. I reproduce it as it shows how many Latter-day Saints at the time not only opposed, but even condemned as an intrinsic evil, marrying non-Latter-day Saints (Smith even imputed a “bastard” status to children born in such unions[*]):
I was very sorry to
hear that Lydia was about to marry a gentile. Nothing would grieve me more,
after spending my whole like, as President Kimball did—for the Cause of God and
the restored Gospel, than to have one of my own children marry out into the
world. In fact, I look upon the marriage of a Latter day saint with a gentile
according to the gentile forms of marriage—and there can be no other form of
such unions, as fashionable or legalized prostitution
<cohabitation> and their children as bastards, for the Latter day saint
should know better than to do so, it is not Marriage at all—in the eye of God
who has spoken and revealed himself and his truth from the heavens in our time,
sin against light, (when the whole light is sinned against) is the sin against
the Holy Ghost, but there are degrees of light and degrees of sin, as there are
degrees of punishment and reward. (Richard Neitzel Holzapfel and David M.
Whitchurch, eds., My Dear Sister: Letters Between Joseph F. Smith and His
Sister Martha Ann Smith Harris [Provo, Utah: Religious Studies Center,
Brigham Young University / Salt Lake City: Deseret Book, 2018], 248. The "Lydia" mentioned is Lydia Holmes Kimball, a daughter of Heber C. Kimball and Lucy Walker. While the union was delayed, she would eventually marry Frances Xavier Loughery [12 June 1875], a non-Latter-day Saint)
(*) Note: I do not agree with this assessment, so any Latter-day
Saint who is married to a non-LDS and have children, this is not an attack on
you. I thought this was interesting, however, as it was written while the
practice of plural marriage was still in effect among members of the Church and
also during a time of strong opposition to the Church due to such.