From what I've seen, it's usually non consensual when it comes to female characters too. Plus that's part of the appeal, isn't it? Having your limbs restrained by tentacles and rendering you completely helpless and immobile, then the tentacles can do whatever they please to your body whether you like it or not...
You know what, i'm just gonna stop here.
There is a lot of non-con for female characters as well but it's become more common to have tentacle monsters that ask for and can give consent with said female characters.
I remember there was at least 3 whole tankobon where the only storylines were involving that tankobon's FMC's relationship with her tentacled's bf, although I only own one. There's also been a bit of a resurgence in consentacles as a genre, but it's still mostly female characters with the tentacles.
For comparison I've only ever seen consentacles content with male characters in single page artworks. I've never come across a full doujin or published eromanga, game, etc with a male mc.