Love them or hate them, tentacles are a fascinatingly sordid subject. Here's my contribution to it - three pluses and five potential minuses to try and explain the cult-love and general stigmatization towards tentacles.
I'll declare my bias up front - I like them. And personally my main reason has nothing to do with the tentacles themselves, it's what they're
used for - an extreme powerplay, as opposed to rape. And in that one sentence is the main reason why I believe many people love and hate it.
PLUS & MINUS: Powerplay vs. Rape
Now this has already been addressed, but it is an important distinction - tentacles and hentai are not
usually realistic rape in any way (massive exception to guro and vore).
I have no illusions that they glorify rape, and for that grounds alone I would never fault someone for disliking them. But the way tentacles and other 'rape' hentai are presented isn't realistic rape - the victim starts out saying "no," but eventually learn to love it. That is in no way realistic, as a) women can only reach sexual climax when they are emotionally stimulated and b) rape has nothing to do with sex, it's about malicious and destructive degradation and domination.
Comparing her saying "no" then loving it to real rape, and what this says about rape, is absolutely disgusting, and grounds enough to hate it. However, comparing this to an extreme powerplay, is why I (guiltily) enjoy tentacles, as they are used to achieve a complete physical, emotional and mental domination of an only psuedo-unwilling partner, using the fantasy of a plant/creature
designed to be inhumanly fantastic at pleasuring it's partner to force them to experience more intense and simultaneous pleasure than is possible in the real world, all to completely dominate them. It is (or at least should be) an extreme sub/dom fantasy with some really nifty tools. A fact a lot of tentacle works seem unaware of.
PLUS & MINUS: Failing to Take it Further
I am honestly surprised more tentacle porn/fiction doesn't seem to be aware of this sub/dom component, as very few (but in my opinion, the best) tentacle works naturally extend on this powerplay, using things like teasing, withholding orgasms, forced orgasms, etc, creating a slow but steady build of pleasure the victim has no control over towards a completely mindblowing climax. Tentacle works that just shove it into an instantly willing subject is, imo, completely missing the point, the reason why I believe a lot of people see it and rightly go 'meh.'
There are also simpler reasons why people dislike it such as:
MINUS: Tentacles Aren't Human
We naturally recoil from anything that's too different from us as humans, most
especially when it comes to sex. It's a basic survival instinct - your mind and body looks at the possibility of bearing children from an inhuman creature (or an inhuman looking human, which I'm deliberately not defining further here as that will open up a massive derailing discussion) who, genetically speaking, will
not be likely to produce strong, healthy children. So naturally your mind and body make you disgusted, filling your head with words like 'slimy,' 'gross,' 'wrong,' 'unnatural' - highlighting with disgust every way in which a tentacle plant/monster isn't human so you won't have sex with it and produce genetically poor children. Basic Darwinism at work.
Then there's always:
MINUS: Penises
Tentacles are 99.9% of the time, phallus's. You are essentially surrounding the victim with penises, sometimes only thematically, sometimes visually explicitly as well. Then a lot of tentacle porn seems to think: "Well if the girl's surrounded by penis's, obviously she needs to be covered in semen as well."
I'm not saying that if you don't like tentacle porn you're homophobic - just that if you're a straight male and don't like the idea of a woman being surrounded by pseudo-penises, that is entirely fair enough. However, I would argue that tentacle works that treat tentacles as human penis full of human fluid are, again, missing the point - it's meant to be domination through pleasure, not messy degradation. But this is also linked to my very last point:
PLUS & MINUS: Censorship Laws
For those that aren't aware, all Japanese pornography must censor all male and female genitalia by national law. A loophole is tentacles, which can be completely penis shaped without being classed as human penises, and thus
do not need to be censored. As a result, a lot of Japanese porn limits it's censorship by using tentacles instead of men. And as a result of that, a lot of tentacle porn is Japanese.
And Japanese porn is often different to Western porn. Sometimes worse so for us - it can be
much more obsessed with waste products, incest, siblings, things that we as Westerners find completely abhorrent, rather than just a little bit controversial. It's a cultural thing. (Also I have no idea how strong the Japanese taboo towards waste products is so I apologize is I'm overgeneralizing here).
But the last thing I will say is that Japanese porn can sometimes be
better than Western porn as well. Animated or drawn hentai allows for there to be an engaging story, believable dialogue and convincing acting
without having to rely on woeful pornstar "acting." It also allows the use of fantastic, impossible elements like magic, machines, super-drugs, gender-changing, futanari's, and yes, tentacles, to show sexual fantasies that go beyond the realm of what is possible (or at least feasible) in the real world, showing human pleasure and emotion in situations beyond what humans can readily experience in our everyday lives. This cuts to the very core of porn and adult-fiction (and even fiction in general) - fantasy, here uninhibited by the limitations of reality.
So, that's my small essay on why I enjoy tentacles, and a whole host of reasons why I completely understand anyone that doesn't. I still maintain that many people who dislike tentacle works may not have seen a good one yet, but again, there are plenty of reasons other than that to hate it.