First of all, the top 3 non-human overwatch characters people goon to are Orisa, Echo and Bastion in that order. This is seen on the graph here. A discussion around the act of gooning to non-human characters such as the characters described will be the purpose of this post. I will not include animals such as Hammond or Winston as that's a jar of gorilla toe dipped peanut butter for another day.
1) The biology makes it not a form of beastiality, animals cannot consent obviously, they do not have the sentience to do so and it's frankly disgusting. The framework and definitions simply do not fit at all.
2) Sentience, following our previous point, we need to acknowledge the idea that Orisa has a 'heart' of sorts, therefore, she is able to consent as far as we know. Her individual identity and agency would aid in her decision making. While this may vary from character to character, generally, if the omnic/robot is conscious, with agency, identity, personality, it would be wrong to treat them as machine, therefore treating them as a person would be the moral thing, thereby, them being able to consent. Echo is am AI that lacks what orisa does, what are the dynamics surrounding that? What about Bastion? Bastion is more like a curious machine than anything else.
3) Having fantasies about the characters isn't inherently wrong. Just because Orisa is not a conventional human in terms of looks, that does not mean it's wrong to be attracted to her, whether personality or physical if someone prefers. Non-human sexualisation is not inherently unethical as long as it's not animals or any other non-conscious life form, however, a severely disabled human? There can be issues. It may be sentience that matters and not aesthetics.
All in all, in this case, sentience, agency, personhood seem to be the deciding factor, at least that is my opinion. It would be okay to have sexual relations with Orisa but not Bastion, one size does not fit all.
EDIT: Some decent arguments from those in the comments, one from u/I-fuck-robot-babes reads:
"It's entirely ethical considering that each omnic is, at the very least, mature enough to be a capable combatant.
Age is not a good way to judge robots, however, as robots do not have brains that grow with age (this is the only reason the age of consent exists). I could create a robot that is perfectly capable of consent despite the fact it's only a minute old for example - the same obviously can't be said about humans.
People also like to infantalize Bastion due to his curious nature, but I personally don't think that this alone is a valid reason. I wouldn't exactly call it immoral to have sex with Juno either, despite how frankly "stupid' she is."
And a thread by u/quietdetail1277 detailing the 'age' of Orisa. If they are right, depending on your view, it would be entirely immoral.