I think it's possible that we never really can fully 'know' our bodies. Some advice on the topic is probably a little misleading or unrealistic.
Many types of touch on different parts of our body aren't going to feel the same every time, and you're right, I am going to say the brain can't be separated from its role in lavishing bodily reflexes with ideas, concepts, beliefs and memories. Trying to do so is going to be nigh on impossible!
I think some of the advice you're referring to is perhaps more aimed at people who've not considered that their body is their own to explore, or have been nervous to look for patterns that will help them feel better, or fearful it could highlight issues in their partner's approaches sex. My belief is that all this exploration happens in fuzzy the presence of conceptual or emotional associations, not instead of, or seprable from.
Recognising that nipples are a part of the body which for you sometimes feels good, sometimes don't is very much the kind of observation I'd some someone would come to if I had suggested listening to their body. So there's no need to worry there!
The way you express yourself during sex doesn't sound inauthentic to me at all, There's nothing particularly authentic about a bodily reflex compared to an emotional response, or any cocktail of the two, they are both just reactions to stimuli, and sometimes a moan is just a moan; we felt like we wanted to and then did it... which can be pleasurable in itself.
Unless you are saying to your partner "My nipples always feel exactly the same and there is never any variation", I don't think you're lying about anything!
"In between two tall mountains there's a place they call lonesome.
Don't see why they call it lonesome.
I'm never lonesome when I go there." Connie Converse - Talkin' Like You