Personally I dislike it when pointless sexualisation turns up in films or TV shows (not just anime). I feel it cheapens them and makes me feel uncomfortable if I'm watching it with family or people who aren't close friends.
If I didn't start watching it because I wanted to watch something titillating, chances are I don't want to be titillated at that point in time. That doesn't mean that I never appreciate being titillated, but I will only feel uncomfortable if I feel content intended to induce titillation is being "forced"* upon me.
The second issue comes where the sexualisation is actually in some way exploitive or offensive. This isn't as much of an issue with comics, video games, prose fiction, poetry, or animation as it is with live action film, TV, or of course theatre as the characters are not being portrayed by real people. I personally found the Luc Besson film Leon somewhat offensive because I thought that it depicted a character portrayed by a twelve year old Natalie Portman in a manner which could be fairly construed as sexualised (all though most people seem to take no issue with this for some reason), but I might just go through my usual "this cheapens the work/makes it a bit uncomfortable to watch with family" routine if the same character was depicted the same way in an animated film.
I may still take some offence at the work feeling that it is OK to sexualise a twelve year old to begin with, but honestly, that gets more into the territory of "is the work suggesting it is OK to sexually abuse children?" rather than "this work is directly exploiting a child."
Obviously, any work** which suggests that it is OK to sexually abuse anyone of any age should be harshly criticised, and may very well be considered obscene by any reasonable definition, but this does not mean that such themes should necessarily be avoided. If a film can sensitively deal with themes such as sexual abuse then that can be a very good thing, raising awareness of the needs of victims and the actions of perpetrators.
*Yes, I know I could just not watch it to begin with, but in most cases that would be a massive overreaction to what is ultimately usually only a minor part of a work.
**Some older works may be treated more lightly in these regards in the same way that I can forgive Help! for being an extremely racist film when couldn't forgive a film that came out today for such things.
TL;DR, sexualisation cheapens works artistically and is awkward to watch if you don't want to be titillated, only 'yes'* ever means yes and only when it comes uncoerced out of the informed mouth of someone over 16 years old, and Senjougahara is best girl.
*As opposed to the absence of a 'no'. Words such as 'yeah', 'that's cool', or 'I want to' may be substituted for a 'yes'.