Somewhat, depending upon the artist and their mindset. Some like showing mindbreak/domination where the victim comes to love abuse of all kinds.
OTOH, disgusting as it is, rape (for example) can cause orgasm in the victim. Doesn't mean victims enjoy being defiled, but rather stimulus response causes it.
The hearts when shitting is pretty much yu canon in that the euphoria is from the purging of bad memories, increasing easiness.
Why are hearts usually placed with bodily functions,actions and harm? I don't entirely get it.
In this case it's mostly because it's shinmai okyashas artstyle. If you look at shinmai's past works they all have hearts everywhere even when abused. Shinmai's yus are also usually very cute even after being tortured and ripped apart and such and usually shinmai would only draw ugly yus after they die. I think this is his/her way to tell us that yus look the cutest when they're suffering. The hearts are probably supposed to symbolize how for us their screams of pain probably sounds really cute.
I thought when an author used hearts was to accentuate how ridicoulous the whole thing is? I mean, look at Takumi and his sparkles Even dead yukkuri would sparkle if he didn't use the black aura of death