The parties to any proceeding instituted in consequence of adultery, and the husbands and wives of such parties, shall be competent to give evidence in such proceeding; provided that no witness in any proceeding, whether a party to the suit or not, shall be liable to be asked or bound to answer any question tending to show that he or she has been guilty of adultery, unless such witness shall have already given evidence in the same proceeding in disproof of his or her alleged adultery.