Reading the first half of this just makes me a little sad. There's nothing wrong with consensual S&M-ish relationships, but with the operative adjective missing, I can't help but feel really bad for Yoshida who is in fact a really nice guy, and is sticking on through constant light abuse (both physical and emotional) out of love and loyalty.
It mellows out a lot in the later parts of the series, thankfully; there are hints of a "healed with the power of love" plot around the edges, and the aspects of an abusive relationship mostly melt away, so that's good...
But the problem never gets directly addressed, and it definitely feels like I was supposed to think the stuff in the first half was "funny and cute" instead of "troubling and heart-wrenching".