I think it's a nod to the end of the manga. (I think Usagi's comment about feeling a new star born inside of her is implying that she's already pregnant with Chibi Usa.) Pregnancy is often a cause for marriage, rather than only the result of marriage, in Japan. It does sound quite young, but I think that young motherhood gets romanticized in fiction fairly often. Despite her immaturity when the character is first introduced, she does have goals that are related to forming a family, so it doesn't seem to be a stretch (to me) that Usagi would choose to have a child as soon as she was out of compulsory education. Thankfully she seems to have a stable relationship and winds up ruling the world and all that. ^_~