Actually, they are two closely related things. ‘Incapacitated’ means that you cannot walk or talk, or focus your eyes, or think, or do anything else requiring cognitive ability, including have sex. The only difference between the two is that you can be ‘incapacitated’ and still have your eyes open and when ‘unconscious’, your eyes are closed.
The man may have been drunk too, in which case he would have had ‘reduced capacity’, but he could not have been ‘incapacitated’ because if he had, he would not have been able to take his pants off and insert his penis into the woman. Incapacitated means NO CAPACITY.
I believe I am beginning to understand your confusion. You say “ That was my reply to your assertion that if the guy was able to perform, he could not be incapacitated by alcohol to the same degree that she was.”
There are no degrees of “incapacitation”. Incapacitation is total. If someone has lost partial capacity, you don’t say they are incapacitated, you say they are impaired, or partially impaired.
“Again, he may have been as drunk as she was. Why then would she not be equally liable for rape?”
This may be true, and if they were equally drunk then neither could have been incapacitated. In that case she would have been able to give consent, and neither would be guilty of rape.