i think that is the issue- who gets to decide what is an understandable and valid reason? I believe it is the woman.
I agree - it's one of those things that no-one can know unless you have been her ... there is so much tied up in such an intimate thing it is impossible to fully know how someone is feeling unless you are them.
And a lot of times, I would guess that a woman herself might not fully understand why it is that way. I went through a period a few years ago where rather than just being lazy about sex, I was actively avoiding it, and it took a long time to even start to work out why & to resolve it.