Putting together Reimu's meandering story, apparently the gist of it was this. With winter approaching, they were all set to begin hibernating when Marisa suddenly said she wanted to refresh. Of course, thinking of the future, they really ought not to have been making children, but despite that, [Marisa] practically forced her to refresh, [Reimu] said.
Thinking that Marisa wanted children, Reimu had reluctantly consented, but when once she could feel the children growing inside her body, she was able to take it very easy. However, Marisa had only wanted to refresh--she hadn't particularly wanted children, and moreover, since they jeopardized their ability to survive the winter, [Marisa] rejected the children.
Thus, the action that Marisa took was to body-slam Reimu, trying to somehow kill only the children. Reimu was unable to stand the thought of the children they had just conceived being killed. Furthermore, she perceived that her own life was in danger, so even with her heavy body, she ran away, and those were the circumstances that had led up to this point.
It was already too late for her to prepare for hibernation on her own. Thus, she sought human help to somehow get by until spring, even if only the children survived. If letting the children take it easy made her become unable to take it easy, that was fine. It was a reckless plan, thinking only of the children, but apparently it was now the only [option] left for Reimu.