Arthur isn't a pedo. He likely understands that he can't be single forever but does not want to be in a relationship with a minor so he told her that probably console her.
Please do not talk about him like other isekai characters e.g rudeus 
The problem would have been if a 40+ year old man accepted a 12 year old's feeling.

Guys, I get him too. I literally drop out of uni because my mom was scared to leave me in another state and told me to try again next year. Did I like it? No. But I know where her heart was. She wanted her child safe in a place she could run to me anytime I'm in trouble. Being mad at your parents is one thing but treating them like this if they only have your best interest at heart is horrible.😐