I think the key word to note here is "bribe." Bribing is not a transaction, contract, or agreement, it is a form of coercion. So this could become an ethical dilemma. If you lure your partner with treats or scratches in the aim of instigating sexual activity, the motive of that behavior is the source of the possible dilemma.
Let's say your treat/scratches/any other form of persuasion is intended to lure your partner into sex without regard to the concerns of your partner. Then I would say yes, you are taking advantage of how you know your partner will react to treats or scratches and using that as a means to initiate sex, without considering the thoughts and feelings of your partner. An example of this might be offering a sex addict money for sex. You understand how that person will react, and you use that reaction to benefit you, rather than take into account that person's thoughts/feelings/autonomy in the matter.
However, let's say your treats and scratches are intended to entice your partner into sex, as a means of initiating sexual activity, and you are considering your partners feelings and are simply initiating sexual intimacy. This would be no different than kissing/hugging/tickling your partner into sexual activity, foreplay isn't unethical right?
I think the important thing here is to consider the intent, if your intent is to bribe and coerce your partner - unethical, however, if you intend to take into consideration your partners feelings and are simply engaging in intimate behavior - ethical.