I don't think it's quite the same thing as the normal obliviousness you get in most of these harem isekais. For one, the Ring King and Hime are in a relationship and have acknowledged their love for each other.
What's happening here is he's wanting to wait until it "feels right" for them to have sex. They haven't "built up" to that point yet.
I can understand that feeling... and also that it's mostly bullshit. But the fact that it's bullshit is really only something you can see after you go through it. For romantic types like these two, losing your virginity seems like a much bigger deal than it really is.
Unfortunately, since this is a manga I don't think it will feel like a natural progression when it finally does happen. I haven't read ahead, but I predict that he'll hold onto his virginity until he has all five rings, and then wind up having to have sex with all of them to gain the power to defeat the main bad guy. I'm guessing a chapter or two on his first night with Hime, then a chapter where the other four happen all at once.