The taboo is still there. However, it is not decided meerly by the surnames. Even one single surname (Yi, or Lee), has many different "Houses", usually noted with originate places. For example, "Kyung-joo Yi" (Yi house from Kyung-joo location) or "Duk-soo Yi", etc. While they have common surname, they have different ancestors.

In Korea, it WAS prohibited by law to marry a person from same houses (same surname and same originating house, called 동성동본, in Korean). But the law has been relaxed (in 90s) and it is legally OK to marry a person from the same surname-house. However, it is still illegal to marry someone closer than 8th-chon. ('chon' is a distance within lineage. Father-me : distance 1. Grandfather-me : distance 2. Sons of my parent's brothers : distance 4. Anyone who has same grand-grand parent is within distance 8 (8th-chon), and cannot marry (legally).

I'm pretty sure that Yi Ch'ang-ho and his fiance is not in the same house. While it is not illegal to marry someone from the same surname-houses, it is still very rare case in Korea.