i like the series in that it seems lighthearted and fun, the goddess part is interesting. But this chapter is disappointing. In general, as someone else said the idea of pledging to someone you don't even know sounds weird. The bigger disappoint though is the Ein / Luna choice and not because of a preference but to me it closed doors and took away the chance to go down some different paths. With the Luna choice I'd bet it becomes basically, most guys hate him and want to challenge him, after all he got the prize. Also I was hoping the story would go down the path of no cred to proving himself and helping Athena. With Luna's choice, while it doesn't immediately erase all issues it will still add a minimum level of respect even if they don't believe it is deserved. To me it really seem like the author chose Trope 13 and when asked if any customization was needed said nope, we'll just drop it in place as it and we're good to go.
With Ein you could set up some situations that are a little more interesting and at least not as quite as much a trope. Aside from just the refusing the noble it could set up some interesting side stories. First it feels like it fits the story a little more, Athena was not really known, while Ein doesn't seem to have that problem, appearing to be more in the middle, she is not likely to give him the support and power of Luna, it would made a more interesting story of watching three like that grow together. Additionally, you could have Luna then pick Alice and I'm guessing she would push Alice into a battle with dickhead and it would carry more weight because of the slight. Additionally it would have put Alice in the position of being the second choice and she lost to her friend so you could play up conflicts there. It just seems like there are so many more possibilities that don't blindly follow a trope as much.
And I used dickhead for the name because whether I wanted Ein or not he was in the middle of making an oath to her and then makes an oath to someone else. If he had even said I was making an oath to Ein, I can't be your knight and Ein said no be her knight I was just apologizing for the other day not asking you to be my knight, then ok. But after you basically beg for someone, she comes up to help out, you start your oath and then just drop it, yep, dickhead fits the character and poor writing from the author from my perspective.