Okay, so. Urabe doesn't want them to kiss and hold hands or even hug yet because they "Don't yet understand each other's feelings." And they do later hug and stuff. But it's heavily implied that those are rare circumstances and they have yet to become comfortable with their bodies. Great. That is the premise of Tsubaki's frustration and the driver of the character development. Each chapter they seem to get closer emotionally and are preparing each other for physical contact at some later point in the future (maybe when they graduate college). That's what makes the story interesting: a slow, logical progression of emotional and physical intimacy that will ultimately lead into sex in probably the last chapter.

When suddenly...

Volume 4 comes around and she's totally fine with him fondling her boob. Granted, it is with her permission. But, I mean, really.... This level of intimacy is never even approached or mentioned after these few chapters; it just seems like a poor attempt at fan service even though we see Urabe in more compromising positions earlier. It's almost as if these chapters never happened and were just inserted to drum up interest... I still love the series, and consider it one of my favorites. I was just a little disappointed by this seemingly out of place groping. What are your thoughts?

I see your point about chapter 21...

But I disagree. The two of them have been growing closer and had at least one serious near-miss kiss at this point in the manga. Urabe also puts a lot of stock in dreams and such, so when Tsubaki mentions touching her breast in his dream, I can see her thinking that it is some sort of sign. (Of course, she didn't hop in bed with him wearing a doll on her head after that earlier dream, but...) This chapter also sets up the state of their physical relationship for the few arcs that follow. They were consistently getting closer and closer to the type of emotional connection that would have lead to more kissing, embracing, and such things. Urabe had even hinted that they would be doing more of that type of stuff when they visited his mother's grave together, then she allowed him to touch her breast, they both were uncomfortable with how things happened. Urabe was disturbed, almost went to the scissors, then seemed to enjoy how it felt, dropping the scissors. But is also made her cry, which totally messed with Tsubaki's brain! He made the girl he loves cry! What kind of monster is he? At that point, I think they both took a mental step back from where they had been heading and put their carnal urges on hold, not wanting to do something that the other was uncertain about and take them beyond their comfort zone again.