If Rei were a guy, I think people would still be reacting the same. Except we'd be pissed that it isn't yuri and we're all yuri fans. The story is set up as a light-hearted fantasy comedy so far. It's not meant to be taken as a realistic portrayal so the story presents the behavior as innocuous, plays it for gags in an instantly recognized comedic form (straight man/funny man) rather than takes seriously, and uses it to show the chemistry between the two characters. The story is going at lengths to show it is not problematic
Imagine if it were Sein doing this but Claire was still responding the same. He would come off as an effeminate, angsty puppy dog without malicious intent. If he came off as an incel, however, it would feel totally different
Apart from that, many isekai works have abysmally sexist relations and interactions that don't get any outrage, so I don't think people would react negatively if Rei were a guy. It's why Sexiled and JK Haru were even written