
Well everyone has their first experience with love, and I'm pretty sure this is both of their first times. I think Fumi is the counterpart to Sensei, because she has knowledge on love from loving her parents. Sensei doesn't seem to have that experience. The author's intentions have been to build Sensei's character by teaching him love, and build Fumi's character by having her grow up and learn to rely on others.
Yes this is illegal but I think the author has made it pretty tame and on the legal side of things. I'm just gonna use my knowledge of laws in the US, but technically there hasn't been anything that Sensei would get arrested for. Had they had sex, it'd would've been charged as statutory rape or even worse.

In Japan the age of marrying is age 16 so it is not illegal. And a lot of the times men are older than the females which is also common. These two are learning about love together and even though sensei is 29, he acts much younger because he is inexperienced in a lot of things in life. He had to grow up fast. We all know he doesn't know how to take care of himself. So he is finally getting that chance to experience things he was never able to before but with Fumi.

You have to remember that Fumi is very mature and responsible for her age where sensei is immature since he didn't experience most things in life that teenagers experience. I am 12 years older than my husband and now we are married 25 years so age doesn't make a difference as long as love exists. Sensei won't have sex with Fumi while she is a minor so he is handling things maturely.

In few years Fumi would reach sensei's age, so it would be more legal. (Sensei doesn't do anything until she would be of age anyway)
If they still gonna love each other after years to come, it's fine.
I know someone that is 26 yro and she has a kid of 8 yro that she had with her 1st marriage. She's now happily remarried with a guy of 60 yro. They are very happy with each other. And the kid likes him.
So age doesn't really matter with love so long you feel happy and comfortable with each other.
Though personally my age range preference is more or less 5 years apart. If Cupid ever comes my way that is T^T (Forever alone)
Oh and my parents age gap is 10 years, but they are married for many many years.
I love this manga..
But..in reality.. would a relationship between a 16 year old and a 29 year old really be long lasting?
Firstly, its illegal.
Secondly, a teenage girl with no life experience regarding love whatsoever and a guy who cannot for the love of god be interested in romance wouldn't just suddenly click..or would they?
What do you guys think?