That’s indeed true. Well, Belldandy has some experience, although Keiichi’s going to be her first male partner (she only had Lind so far, so all her experience is with goddesses). Also, Skuld isn’t too experienced either, since she just got a boyfriend and at this point, Keiichi might actually have more experience than her.
It is also the reason why I’m not going to directly involve all characters with Keiichi, to keep the story a bit more plausible. For example, Skuld already has a boyfriend, yet she’s there to spend some time with her sisters, and Lind is mainly involved with Belldandy (who in turn will become involved with Keiichi). And both Hild and Ansuz have a lover of their own, but since their daughters are involved with Keiichi, there might be a few moments with him too, eventually.