How to distinguish soft and hard no? What is caveman sex?

There's more than one way to skin a cat. You can go full caveman as described by /u/Sepean, which has worked very well for him and fits with his likes/dislikes in the bedroom, but in your specific case you should read through the Sex God Method and think about what you like and need in bed.
 
SGM talks about Dominance, Emotion, Variety, Immersion. It talks about focusing on your own enjoyment, and giving your partner pleasure and orgasm on your own terms. Don't neglect her, just fuck her right.
 
Emotion
If it's all about the physical sensations, then she's better off with a vibrator. Emotions are what make sex so satisfying. Emotions are why masturbation, oral and manual sex don't give either of you that deep sense of satisfaction.
 
Variety
Instead of playing the same song in Pussy Guitar Hero every time. It's time for sex. Okay, ready, 1, 2, 3, kiss, rub back, rub butt, rub front, kiss neck, go down on her until orgasm, win at sex! Two orgasms, high score!
Doing the same things every time is boring. You need some degree of dominance to avoid that.
 
Immersion
Extraneous thoughts like whatever she was doing in the kitchen last night (your other post) will kill the experience. You were immersed in a sexual encounter because you woke up from a sex dream. She was having a dream about whatever was in the kitchen.
 
Dominance
The most important. Why? You can't maintain EVI without the D This is caveman sex - sex on your terms. Take pleasure in it. Give pleasure when it suits you.
Calling the shots also allows you to maintain control over your orgasm, so you can prolong the experience if desired.
 
Daniel Rose says that you need incorporation and balance of all four components. He's right. The specific balance depends on the couple's preferences, the phase in her cycle, and a host of other factors, but if you neglect one then sex will suck.
 
Last night you were missing Dominance because you didn't pass the LMR shit tests. You were missing Emotion the second time, and maybe the first. You were missing Variety because what you describe is a standard, boring, in-bed, after all the chores are done, massage-then-fuck sequence. You were missing Immersion because you didn't pass the LMR shit tests. I don't know if you were using kino and playing with her earlier in the day, so I'll give you the benefit of the doubt here.
 
Edit:
Distinguishing hard and soft no isn't always straightforward. "No" means no. If she's puking and has a fever, any answer probably means no. Anything vague that seems like a poor reason can be treated as LMR until she gives a more clear "no." Kids might hear, gassy, have to get up early, you didn't start the dishwasher, lights are on/off, whatever.
 
LMR is anti-slut defence as described in TRP, but there are many other reasons for it. She is consciously or unconsciously trying to decide if sex with you is worth it. Are you worthy of her eggs?
 
Shit tests let her decide if you should be allowed to approach. LMR lets her decide if you should be allowed to go all the way. A hard "no" means no. Body language, context, and the words all matter to some extent, but words are the least important of the three. Use your judgment.