I do understand the plot, although there's a lot of translation mistakes in chapter 5.5, and quite a lot of the sentences don't make sense. But it has nothing to do with quality of this work in general.
This story is a baka comedy. It's silly, it's funny, it's almost a yaoi parody. Well... not almost, it is a BL parody. I realized it just now, while writing this comment. I guess the humour is the best selling point of this manga, maybe even better than hot kinky sex. Our main character's illness do not exist in real world, what's more it's completely ridiculous, but it's okay, since this manga was created for fun. Characters are perverted but likeable, they are considered of others, kind and understanding, also good looking and smart. The fact that they have to deal with something so ridiculous, so unearthly like Tsutomu's illness with all their wisdom and seriousness creates a funny gap (I guess it's called MOE and it's a basic gimmick when it comes to the Japanese sense of humour). Even Tsutomu's age is a part of parody - there's a belief in Japan that if a male virgin turns 30 years old he will be able to use magic. Anyway I really like the inner dialogues in Tsutomu's head and Haruka's dedication to his beloved one. Their actions, especially going overboard in many situations, are also a part of parody, but I won't bring examples anymore cause I don't want to ruin your fun.
The art is good, through I needed some time to get used to it. There's a lot of hot and kinky sex and even toys for adults (BL parody without sex toys? impossible!). The plot is good, entertaining, and even if it's based on a silly idea it's still far away from being dumb, surprisingly this manga has shown it's share of worldly wisdom by the end of the story. Characters had grown and learned that the best cure for any and every trouble is real love. Is there a better ending? XD