All dogs seek it out with a behavior that's not hard to understand if your aware how they communicate: even if their personalities, temperament, etc may differ it's all the same cues throughout the species, it's a shared language for them simply put: although most non-zoos would try to argue that a dog cant ask for sexual contact from humans, and try to assume we can't translate what they want with the right knowledge.
They can never consent verbally, duh: making that as conditional like you would with human courtship is unsound and illogical. to project 'verbal' anything on a situation that's different in circumstances of communication makes no sense. They're not human! but that doesn't make them incapable of communication altogether. But if one really wants to make verbal agreement a requirement for sex then I guess a human that is mute or doesn't speak a certain language cant express obvious arousal through body language. Right? Wrong. After all it exists in all mammals. Fact
Any one ever who has owned a dog knows they can express desires, as well as their personal preferences. If you doubt that then you really do underestimate the intelligence of animals
Masturbation is part of normal sexual behavior for both altered (spayed or neutered) and intact dogs. Both male and female dogs mount other dogs, people and objects, and will continue with that behavior if not scolded for it simply because it feels arousing to them.
Most people don’t realize that humping behavior isn’t limited to intact male dogs or 'dominant' behavior, nor that neutered males can achieve erections just like intact males and can show interest in sex even without their gonads. SEXUALLY MOTIVATED MOUNTING and masturbation are usually accompanied by “flirtatious” body language and friendly courtship behavior (tail up and waging, or held in a comfortable normal relaxed stance, might be accompanied by broad full strokes {low but not tucked}, ears rotated backward {not pinned back flat}, licking and pawing. Bowing {big stretch ?} is also a direct indication that a dog is comfortable in a situation and inviting you to continue. As well as Tail flagging, sheath erections {accompanied by urge to mount} and even sudden panting not due to temp or exercise can be signs of obvious arousal. Etc) and is not accompanied by any dominant or tense stress related body language.
Rule of thumb for newbies: If you can't determine a dogs body language efficiently; without any doubts in your ability too do so or if they are not making an obvious direct move on you. Don't attempt sex. Especially don't attempt sexual contact if you try to stroke a dog and they don't respond with humping at all to your advances. 10 times out of 10 that means they are not interested! I can not stress the importance of knowing k9 body language thoroughly since its the only way bestial/human consent is POSSIBLE!!