Children born after one hundred and eighty days following the celebration of the marriage, and before three hundred days following its dissolution or the separation of the spouses shall be presumed to be legitimate.
Against this presumption no evidence shall be admitted other than that of the physical impossibility of the husband's having access to his wife within the first one hundred and twenty days of the three hundred which preceded the birth of the child.
This physical impossibility may be caused:
(1) By the impotence of the husband;
(2) By the fact that the husband and wife were living separately, in such a way that access was not possible;
(3) By the serious illness of the husband. (108a)