Rta

Dharmecha, Arthecha, Kaamecha naathicharami - I shall not violate/transgress the boundaries of Dharma, Artha and Kaama, the three basic kinds of Purushaarthas while married. Why not the fourth purushaartha of Moksha? why is the man permitted to transgress the boundaries of marriage to achieve the last one of the purushaarthas, viz., Moksha?