@tailor31415 More like he's been a pushover most of the time, and the fish was just the topic that started the conversation this time for these two (and led it into the topic of marriage, since he had just promised to be a matchmaker for a fish, just to get her to eat.)
The actual reason the male football fish attach to the females is that females are so rare, that if they DON'T attach to them, they will probably won't live long enough to find another one.
While the educational part is good and it's a pretty fun manga, I really want some background on Koutarou though. On one hand we have a state of the art underwater aquarium but somehow Koutarou still uses an oil lamp on his late night reads.