Whenever an issue of paternity of a child born or conceived during a marriage arises in any civil or criminal proceeding, the presumed father or the mother of such child is competent to give evidence as to any relevant matter regarding paternity of the child, including nonaccess to the present or former spouse, regardless of any privilege which may otherwise apply. No parent offering such evidence shall thereafter be prosecuted based upon that evidence for any criminal act involved in the conception of the child whose paternity is in issue and/or for whom support is sought, except for perjury committed in this testimony. (1981, c. 634, s. 1.)