Section 15. Whoever, having a former husband or wife living, marries another person or continues to cohabit with a second husband or wife in the commonwealth shall be guilty of polygamy, and be punished by imprisonment in the state prison for not more than five years or in jail for not more than two and one half years or by a fine of not more than five hundred dollars; but this section shall not apply to a person whose husband or wife has continually remained beyond sea, or has voluntarily withdrawn from the other and remained absent, for seven consecutive years, the party marrying again not knowing the other to be living within that time, nor to a person who has been legally divorced from the bonds of matrimony.