Why would you draw the line with sexual contact? How is sex any different from any other instinctual behavior like hunting, herding, or guarding?
Because sex shouldn't be a job it should be fun and completely voluntary, nothing should happen good or bad if you don't want to participate because your not really in the mood or don't really enjoy the particular act being requested by the human.
If you want to put it into instinctual terms
If you don't hunt you don't eat
If you don't herd, hunting or keeping the pups from wandering off can be harder
If you don't guard another predator might take your pups.
When dealing with an animal partner it is the humans responsibility to learn what the animal does and doesn't enjoy when it comes to sexual contact, and only do the things that can be enjoyed by both parties. If you introduce the variable of a food reward for an action, then you lose the ability to determine what the dog is really telling you: Are they truly enjoying it and happy to initiate the action because they like doing it, or are they doing it just for the reward.
A sex worker might give you a blow job, it may or may not be enjoyable for them to do, but in most cases they are doing it not because it is enjoyable, but because they get a reward (money) for performing the act.
In a loving sexual relationship though coaxing, coercion or rewards should not be needed or used to get your partner to do something, if they don't want to do that particular thing, do something else they do enjoy or let them be.
As I tried to say at the start of this discussion with you it's all how you see the dog:
Are they your partner, you love and respect and want them to be a willing participant.
Or
Are they a tool you use to get sexual pleasure so their true wishes are secondary to the goal of getting off.