These sort of arrangements were very common in Feudal Asia, though more commonly with male rulers and his female consorts and concubines. The key is that “marriage” is not about True Love in our modern age or in The Princess Bride, but rather a contractual arrangement.
Having firstborn Ming as her official husband gives her children more legitimacy, and also wins Ming as an ally if a future challenge to the throne rises (but that won’t happen, will it?).
Also, by proclamation of the previous Emperor, Tian was banished from the capital. As regent, she could rescind this, but it could possibly make her look immature and emotional to the Great Lords—here’s a nineteen-year-old girl who overturns the decision of a once-admired Emperor, just because she wants to marry the man she’s in love with.
Finally, it sets up the working opening line of a future story:
On his fifth birthday, Wang Xi discovered he had a third father, who lived in the only place Mother forbade him to go.