Namjoons dog always runs off and he takes advantage of that fact (I suck at bios oops)

Namjoon's and Jin's mothers promised that they will marry their first born when their children reach the age of 20, if they (the children) was still single when they reach the age of 20, but they both didn't know that their first born will be both male. ➖ "Apparently our mothers had made a stupid vow and now we both are involved." "Can't believe they actually did it." ➖ Please forgive me if I make mistakes. My English isn't that good. T^T Suggestions are welcome.