My girlfriend of one month was having sex with a frivolous male senior I disliked. (Akari Tsumugi ADN-314)

I've been dating my girlfriend for a month, and I really like her, but we haven't held hands yet. I thought she felt the same way... but obviously, we're not. Although I didn't ask any experienced people for help, he showed me some self-made sex videos and saw my girlfriend's lewd behavior...