You don't need to have a tentacles genitals to "imagine", what you think imagine is? It's human ability to see and feel the unreal.
Tentacles literally genitals with ability to move whatever you want, it can become soft/hard/big as you will, and no question it's pretty long, it can reach anywhere.
90% tentacles videos are rape, they're so gross, so disgusting, but still able to make all girls climax (which is possibly faked when women have normal sex), why? Because it's inhuman ability to bring pleasure (it can also contain aphrodisiac).
If you actually have tentacles, then you don't need craps like gaze, body, hands, environment, situation, and anything else (you don't need but you still able to do them, it doesn't like you'll become blind or amputee if you have tentacles, it's just doesn't necessary). You just need to sit there and let the tentacle do it job. Tentacles strong enough to restrain and lift the body, stimulation any corner, and penetrate all holes at same time. You don't even need other men (which I hate most when another man join in my fun) to have threesome or gangbang.
I can understand if you simple hate beast porn but you can't deny the power of tentacles.