Of course this is open to interpretations, but heres my view...
Ros and Guil 'have no control, none at all' which is reitterating the
theme of fate and destiny in the the text.
Guil, being the wiser, more intellectually capable of the two
chacacters realises at the end that they could have changed their
destiny at some point. They could have taken the initiative and
directed their own lives for a change. So, when Guil states, "there must
have been a time, at the beginning, where we could have said-
no" he has come to this realisation.
Errrr... hope that makes sense