I´ve seen a lot of comments on this topic and even more simply insulting Manabu. Hence the thread.
Before anything I would like us to disregard that the manga would find its end if Manabu would chose Megumi right now.
Personally I don't think that we can judge Mikawa as of now since we don't know a thing about her and her relations to Manabu or Megumi. We only know that she is a genius that easily manages to do what he fails at and that she is beautiful so as to receive multiple confessions. His friend said that she is "to good for us".
From his point of view Megumi doesn't mind him when she is around him letting her guard down and therefore doesn't see him as a guy. He beliefs that he is to bland of a character and doesn't match her. No he actually even thinks he gets in her way of having fun as one can see in the beach chapter. That they didn't hang out in middle school probably only continues to strengthen his conviction.
Although I don't know why he might assume that he is a match for Mikawa. Maybedue to the same intellectual interests?
One also has to consider that MC fell for Mikawa because she can do everything he can't and and thus is perfect.
Furthermore you can't simply choose with whom you fall in love with. Life is not so easy.
Lastly he is getting more consious of her by the day but he probably thinks he is only lusting for her body. That might eventually bloom into love some day but as of now we can't be sure if he is simply horny or fell in love with our lovely Megumi
Sorry for the bad english and I hope for a fun debate