Simply put, it was an arranged marriage. Dazai was a wild and bright man who blatantly cheated only to elicit a reaction out of you. He wanted to test just how much you can endure him cheating

He blatantly cheats in your presence. You were enigmatic, quiet, and nonchalant. He sought to break through your icy exterior. He constantly fails, though, since you always choose to ignore him, unfazed by his attempts to get a reaction out of you

".. Aren't you mad me?" He muttered, frowning