From what I read here, (I don't know about other readers and no offense to them, afterall everyone has their own way of seeing things), i think Yagyu has a very very casual attitude towards romantic relationships. For him, it's like "okay, a random girl asked me out, so I'll just play along. I don't care as long as she is not annoying or clingy. He likes to be in short-term relationships. Although his ex was the one to dump him, but the way he didn't feel uncomfortable or uneasy in front of her, just threw light of how all of this is just a casual thing for him.
And him saying "Satomi, you are my no.1" and all.... Feel (to me) like empty words so as not to rock the boat.
Here I can see Satomi too much invested in this relationship to the point that I feel while reading, that had I been her parent or guardian or elder siblings, i would have been worried about her.
But anyway, this relationship would either turn out to be her true love or a true lesson.
Only time will tell...
Now this is a manga, so all readers are cheering for th.... that's okay.
But, please.... All young girls out there.... IRL... Don't be so invested in someone like Satuomi. You could get hurt big-time.
Test a person's loyalty towards you before accepting him. Same goes vice versa too.
Love yourself first.....