It might've been the setup. Not just the blushing. The scenes were overly setup, just like any other romantic scenes. It gave hints of romance, just like any other stories. That's why some felt uncomfortable. Imagine if Theor wasn't a kid, that would come off as a pretty romantic setup for a ML-FL scene. We get the first crush thingy, but the illustrator or the author did that scene weirdly to come off like that.
I saw redudancy with my comment, my bad. But yes. Theor having a crush on Marian isn't bad. He finds Marian pretty and that's cute. Just that the scene wasn't portrayed right and it gave off weirdly romantic, which is an ick for some.
Idk why but for some reason some mfs be really thinking "oh she blushes too much over a literal child it's weird." Mf yes she blushes but that doesn't mean she fking likes Theor romantically, it's normal for children to have crushes on adult. But as for Marian's case, she could be feeling embarrassed, happy, or grateful for Theor cheering her up and dancing with her.
Y'all need a therapist if you have that kind of mindset.