A man might choose to honor a slave's "no", but that's not really relevant. A man with a drawn gun might also choose to honor a woman's "no". In each case, though, that's more a function of his situational freedom than of hers. Perhaps more accurately, her ability to give unconstrained consent is a function of (among other things) her perceived freedom to withhold it.
I don't see the two scenarios as equivalent, and I'll tell you why.
A man with a drawn gun is presumably not asking for consent. There's a reason he's drawn the gun. Unless it's for self defense ("Okay, now that I've persuaded you not to kill me, do you want to get it on?"), it's probably to compel consent. The implicit choice is, submit or die.
If men were walking around all the time with drawn guns, it might be a more equivalent situation. The imbalance of power is still a given, but unless people are being shot to death over every disagreement, the imminent threat of death is much smaller than in the scene you set. If a soldier in Vietnam had relations with one of the locals, it wasn't automatically presumed to be rape.
A slightly different angle on the question arises from the fact that slave status entailed general legal incapacity. In many jurisdictions nowadays, a sexual act is deemed to be committed without someone's consent where the victim lacks the legal capacity to give consent even if she in fact acquiesces to the sexual act.
Why are you having such a hard time understanding that a slave cannot give consent? All civilized countries in the world have laws against statutory rape because all reasonable people agree that a child is not capable of giving consent to a sexual union that is as unequal as an adult-child relationship. Similarly, slaves cannot give consent because the relationship is inherently unequal.
"Inherently unequal" does not constitute statutory rape, and I don't think slaveowner-slave sex would be in the statutory rape category any more than consensual rich man-poor girl or boss-secretary or president-intern sex.
The statutory rape category has a "legal consent" provision because some people lack the mental capacity to perceive the consequences to the degree required for informed consent. A child is protected for this reason. I don't know of any specific cases, but it may be that a person with diminished mental capacity may be similarly protected.
Even if an adult slave did not have legal standing to refuse sex, if consent was given, I don't think the act would be considered rape, statutory or otherwise.
You're really trying to have it both ways here -- defining the slave as a modern person for the purpose of establishing rape (sex without consent), and then as an 18th-century non-person for the purpose of being incapable of giving consent. If she was a modern person, and consent was given, it wasn't rape. If she was 18th-century property, consent would not be possible, and rape would not be possible, any more than a man could rape a cantaloupe.
I'd prefer to regard her as a person under conditions which might have exerted influence, but did not compel consent. Under those circumstances, if consent was not given, it was simply rape. If the act was consensual, even if influenced by circumstances and an inherent power imbalance, I'd be reluctant to call it rape.
