Because there are manga that deal with men-women relationships in a mature and respectful manner.
There are also manga which, while it deals with "out-of-the-norm" relationships and sexual interests, do so in a engaging manner and end up being fun reads, if silly at times.
Out there are mangas which are so obvious on the specific fanservices and so blatant in pushing them to the reader that they would be "porn with plot" types except they're not pornographic (not that their actual porn counterparts are much better).
Lets's not forget that there are mangas which show unhealthy, creepy and/or borderline criminal actions and habits like good and desirable things to happen, just because their target audience can, and sometimes is, engaging in those same activities. Or not and need a relieve, hard to tell.
And then we've got THIS.