I was reading the http://www.uesp.net/wiki/Lore:The_Monomyth and I saw this line: "...others, like the Bosmer, had soiled Time's line by taking Mannish wives." What does that mean? Are they another type of half-elf like Bretons?

Yep, interbreeding. Probably not to the same extent as the Bretons (humans in general tend to be more mixed genetically), but not wholly pure, either. Half-elf is really a missnomer, since none of them are 50% elven in genetics or appearance.