Will their relationship ever get physical? Most likely very, very slowly. Just recently there was a series of chapters on kissing, so maybe they'll begin casually kissing... but, in short, their physical relationship is not the focus of the story.
Recent chapters around the introduction of the Gate introduce this topic for the first time openly (though somewhat symbolic.) Keiichi and Belldandy have never taken the physical aspect of their relationship lightly, and the reactions of both of them show that. What exactly it does mean is not that easy to say, one speculation might be that it shows that their relationship is clearly not based on physical aspects, but is deep and honest - however, they might develop something on this level. See the article about the Gate for details of the developments during this storyline.
Another thing is, that if we go back to the Peorth arc, we can see that Belldandy mentioned that her wish was to fulfil every single one of Keiichi's desires. The problem? Is that she wasn't ready, and it seems that she still isn't. In other words, it's the common "I also want to do it... but I am not prepared."
Marriage, Family and ChildrenEdit
Will they marry and can or will they have children? Since their future is very unsure since their relationship is unnatural, Keiichi is most likely really scared to make Belldandy keep a commitment that she's unable to carry out. As for children, it's clear that gods and demons don't procreate very often; it seems unlikely that goddesses menstruate, given Belldandy's and Skuld's confusion regarding the subject, so Belldandy being able to have a child with Keiichi naturally seems unlikely in that regard. But may seem possible.
After they finally learn more about each other, in the manga everything after that could be called the stage of "outside forces". Urd and Skuld are opposites ends, since Urd is trying to get the two to become physical while Skuld is trying to keep the two away. Later, they both mellow out, although they still seem to keep their prime directives in order every so often...
From MangaWiki, a Wikia wiki.