Which country became the first in Asia to fully legalise civil same sex marriage through national legislation?

Introduction / Context:
Legalisation of same sex marriage is a major civil rights milestone and often appears in current affairs sections related to social change and constitutional law. While many Western countries legalised same sex marriage earlier, Asia saw its first such nationwide law only recently. This question asks you to identify the Asian country that became the first to legalise same sex marriage through its national legislation.

Given Data / Assumptions:

• We are looking for the first country in Asia to legalise same sex marriage.
• Options are Vietnam, South Korea, Taiwan and Philippines.
• The legalisation referred to is a comprehensive civil law allowing same sex couples to marry with rights similar to opposite sex couples.
• We assume up to date knowledge as of common exam years when Taiwan was widely reported as a pioneer in this area.

Concept / Approach:
The key concept is awareness of global LGBT rights developments. In 2017 the Constitutional Court of Taiwan ruled that existing marriage laws were unconstitutional for excluding same sex couples and gave the legislature a deadline to change the law. In May 2019 Taiwan's parliament passed a bill legalising same sex marriage, making Taiwan the first Asian country to do so. Vietnam, South Korea and Philippines have had debates and partial recognitions but have not enacted full same sex marriage laws at that time in mainstream exam references.

Step-by-Step Solution:
Step 1: Recall news about huge celebrations in Taipei in 2019 when same sex couples began to marry legally. Step 2: Identify that this happened in Taiwan after a ruling by its top court and subsequent legislative action. Step 3: Check the other options. Vietnam has made same sex marriages not explicitly illegal, but it does not give them full legal recognition equivalent to heterosexual marriage. Step 4: South Korea and Philippines have active LGBT communities but do not yet have full marriage equality laws in force at the national level according to most exam time frames. Step 5: Therefore Taiwan is clearly the correct answer.

Verification / Alternative check:
An alternative way to verify is to recall headlines used in many news portals and exam magazines. They often carry phrases like Taiwan becomes first Asian country to legalise same sex marriage. Some sources also note that this move came ahead of similar reforms in other Asian democracies. No such widely publicised statement exists for the other options, confirming that Taiwan is the pioneering country in this context.

Why Other Options Are Wrong:
Option Vietnam is wrong because although it removed explicit bans, it did not grant full marriage rights and therefore is not counted as having legal same sex marriage in the same way. Option South Korea is wrong since same sex marriage is not officially recognised nationwide; attempts to register such marriages have been rejected by authorities. Option Philippines is wrong because it has strong religious and legal debates on the topic but has not passed a comprehensive same sex marriage law at the time generally covered by competitive exams.

Common Pitfalls:
A common pitfall is confusion between softening of penalties against same sex relationships and full legalisation of marriage. Some learners also mix up civil unions, partnership rights and marriage itself. When answering such questions, focus on whether the law grants the title and legal status of marriage to same sex couples across the country. Remembering a few headline phrases like first Asian country to legalise same sex marriage can serve as an anchor for quick recall in the exam hall.

Final Answer:
The first Asian country to fully legalise civil same sex marriage through national legislation is Taiwan.