My spouse had a crush on someone of the opposite sex. Is this grounds for divorce?

Yes it is possible.
There is a precedent in which cheating was acknowledged based solely on one-sided unrequited love.

Her husband was attracted to a Chinese woman who was a co-worker of her company, and ① unilaterally gave her a gift, ② sent her a poem, and ③ contacted her on several occasions with the intention of dating her.
It was held that this act amounted to misconduct in her relationship with her wife. (Suwon Family Court Ansan Branch 2019 Dedan 37022)

The judge acknowledged the breakdown of the marriage for this reason, even though "they did not appear to have reached the stage of substantive dating." In other words, it was made clear that actual dating does not affect the grounds for divorce. 20 million won was recognized as alimony for this unrequited love.

However, the mere fact that she courted the opposite sex will not necessarily be grounds for divorce.
Depending on the sincerity of the courtship, you may be able to defend yourself by saying that it was just a joke/crush and that you have now reflected on your feelings and made a sufficient apology and will continue to do so in the future. In other words, the outcome of a divorce lawsuit will vary depending on the development issue.

This is a procedure in accordance with Korean law. For foreigners, detailed procedures may vary.
Please chat or email for specific details.

Are there any legal issues involved?

Should I negotiate with a Korean or initiate a lawsuit in a Korean court? The law firm where the interpreter works is located here. All of these processes can be carried out NON-STOP through a professional Korean lawyer. Contact us for consultation right now. Professional staff will respond kindly.