I guess they wouldn't give me permission to stay at a female friend's home, but that's understandable.

understandable 是由动词 understand（理解）加上形容词后缀 -able 构成的形容词，表示"能够被理解的"或"合情合理的"。-able 这个后缀表示"能够...的"或"值得...的"，所以 understandable 字面意思是"能够被理解的"。