Okay, there are a few angles you could work in that scenario depending on how much work you want to do and what tone you want to have.
Focusing on the indignity of it all as I think you were considering to begin with is definitely one way to do it. With male-perspective scenes the male ego is always something to keep in mind, whether stroking it or bruising it, since it's so paramount to how men think of themselves and others. An anal virgin variant scene could be especially harsh, focusing on how the character has lost command over himself and can't even say he's being fucked but violated in a deeply unsettling way, tries to distract himself by going over the fight in his head, thinking of what he should have done but eventually, to his shame, he can only focus on the tentacles or fist in his butt and as he tries to resist having an assgasm with all his might he suffers another loss to the sexbot. Another way of approaching that same theme with more experienced characters would be how embarrassing it is to lose to a broken sex doll of all things, how the treatment's giving them a weird boner, and even on the companion at first cursing themselves for being unable to help but eventually finding it kind of hot and rubbing themselves to it a little, the perspective character notices and can either ignore it or awkwardly try to put on a show for their companion.
Or (again, depending on how much work you want to do) you could have a resist scene that focuses on this humiliation and then a give in scene that goes the other way, focusing on how being torn down by a broken machine intent on getting in him is getting him worked up and then writing it as more of a tentacle-monster style scene where the bondage and forced pleasure quickly flip his switch and have him wantonly enjoying it. Scenes like that are all about how the mindlessness of the monster breaks down all social stigmas and expectations against sluttiness and also how being out of control of his own pleasure (and especially having too much forced on him, a sensory overload) melts away any civilized notion of himself till he's just an ineffectually rutting thing just like the monster that's raping him, its equal-opposite.
Putting aside the basics of writing what you're going for with porn is hitting specific erotic triggers. You have to continually ask yourself what's erotic about each element and why. Context is very important so setting the scene is a better area to concentrate on if you're not sure how to make the actual mechanics of it sound appealing.
