According to Rodney Turner, it should be recalled that the Lord's edict concerning the imposition of the Lamanite mark was not limited as to duration and extent:
And cursed shall be the seed of him that mixeth with their seed, for they shall be cursed even with the same cursing. And the Lord spake it, and it was done. (2 Nephi 5:23)
In all likelihood most pre-Columbian inhabitants of the Americas--of whatever original race or culture--eventually came under the edict: "I will set a mark upon him that mingleth his seed with thy brethren, that they may be cursed also" (Alma 3:14-15). If so, then through assimilation and intermarriage, other peoples like the surviving Nephites became Lamanites (Helaman 3:16). In this way the blood of Joseph was scattered among the diverse Indian peoples of the Western Hemisphere (Kimball, ". . . who is my Neighbor?" 277).
That such was the case is supported by many statements by latter-day prophets. Regardless of current theories to the contrary, every prophet from Joseph Smith to the present has declared that, in the main, the Indian peoples of the western Hemisphere, as well as certain Pacific islanders, are of Israel through Joseph (Teachings of the Prophet Joseph Smith 17, 92-93, 232, 266-67; Jessee 324; Journal of discourses 2:200, 7:336; Kimball, ". . . who is my Neighbor" 277; "The Evil of Intolerance" 423). [Rodney Turner, "The Lamanite Mark," in The Book of Mormon: Second Nephi, the Doctrinal Structure, p. 153]
Cursed Shall Be the Seed of Him That Mixeth with Their the Lamanite Seed
Brant Gardner notes that in 2 Nephi 5:23 the Lamanite curse extends to those Nephites who might marry any of the Lamanites. Thus there is not only a geographic division between the brothers and their families, but a moral and religious chasm as well. The question of marriage has become an important one because the Lord has now excluded a significant number of the potential marriage partners. While the preferred mode of marriage would be to marry someone within the same tribe (see Numbers 36:6), that option has been limited.
Marriage in the ancient world dictated the flow of rights and properties. In the case of Israel, it was not only physical property, but a religious inheritance that was passed on. Thus the injunction to marry within the tribe. This preference for marriage inside the tribe was coupled with other prohibitions against marriage outside the tribe:
In the Hebrew commonwealth these prohibitions were of two kinds, according as they regulated marriage (i) between an Israelite and a non-Israelite, and (ii) between an Israelite and one of his own community. - (i) The prohibitions relating to foreigners were based on that instinctive feeling of exclusiveness, which forms one of the bonds of every social body, and which prevails with peculiar strength in a rude state of society. The only distinct prohibition in the Mosaic law refers to Canaanites, with whom the Israelites were not to marry, on the ground that it would lead them into idolatry (Exodus 34:16; Deuteronomy 7:3-4). But beyond this, the legal disabilities to which the Ammonites and Moabites were subjected (Deuteronomy 23:3) acted as a virtual bar to intermarriage with them, totally preventing the marriage of Israelitish women with Moabites, but permitting that of Israelites with Moabite women, such as that of Mahlon with Ruth. The prohibition against marriages with the Edomites or Egyptians was less stringent, as a male of those nations received the right of marriage on his admission to the full citizenship in the third generation of proselytism (Deuteronomy 23:7-8) (William Smith, "Marriage" in Smith's Bible Dictionary, 1970, pp. 376-377)
The prohibition against marriage with the Lamanites comes very early, and at this point is apparently directed at the specific tribal affiliation of Lamanites/Lemuelites, etc. In the future text of the Book of Mormon when the term "Lamanites" appears to become a more generic term, akin to "gentile" it is likely that the specific prohibition was lessened, although it might have remained in place with the lineal descendants of these original Lamanites.
The inevitable intermarriage with other natives of the land, while not the preferred method when the larger population of Israel was available, was nevertheless allowed, and preferable to the more direct violation of the prohibition of marrying within ones immediate and near family (which certainly would have described the original Nephite and Lamanite bands at this point in time). [Brant Gardner, "Brant Gardner's Page," [http://www.highfiber.com/~nahualli/LDStopics/2] Nephi/2Nephi5.htm, pp. 25-28]