Taking a break from Kagami to comment.
I want to preface what I'm about to say with the following: the actual
chapter is well-written. And some parts definitely do enhance the whole story. I can see your reasoning for wanting to keep it.
I think it's the nature of healthy, happy sex in the context of a two-party romantic relationship that doesn't translate well to prose. I mean the actual description of the act. And I think it's this way for two reasons:
1. Prose is so incredibly divorced from anything regarding this kind of sexual experience. Think about it - sex is touch and sight and smell and sound. It's probably one of the most basic sensory acts humans can participate in. Prose is more high-functioning; it's character development and plot progression. These are less-than-basal concepts. And unless your sex scene is doing one of those two things (which, as well-written as yours is, doesn't really change Emi or Hisao, nor does it put them in a particularly different place in their relationship), the actual sex can be implied and still have the same effect.
As a side note, I think the reason some of the more - ahem - gratuitous sex scenes work as well as they do in the VN is because it's not an entirely prose-based medium. Who knows how well this particular chapter might have worked if it were in that format.
2. Tying in with what I was saying earlier, I think in order for pure sex scenes in prose to work (outside of straight-up erotica), they really have to develop character or move the plot along. When that doesn't happen, what we're left with is an almost voyeuristic window into a pretty intimate act between a couple. It's not
creepy, per se, but it does inspire feelings that lead to questions such as, "Why am I here? Why am I reading what I'm reading?" For Emi and Hisao, this encounter is important to them as a couple. As a reader? Eh. I know what sex is. I know most couples have it. And I know, in a healthy situation, it can lead to the strengthening of a bond between two people. All of that could have been said in much less space, and honestly my reaction as a reader would probably have been stronger. A description of their post-coitus disarray engages the reader much more than a painstaking description of Emi's buttocks.
This isn't to say it was badly written. Nor is it even to say that I'm not guilty of some of the same mistakes myself. Just my own observations. Lewd for lewd's sake is fine, but I'm not entirely sure it works here.
Christ on a crutch, I probably sound like a curmudgeon with how much I complain about this story. Trust me, I wouldn't whine if I didn't care as much as I do. Looking forward to future updates.