To be "legally" separated there must be an agreement or a court order. Its meant to spell out who pays for what and to make sure no one cleans out the accounts until a court decides or you two come to an agreement.
The romantic aspects of you two do not figure into it. You are both free to "do" whom you like. The Legal separation does not give anyone permission to fool around nor does it prevent it. If he was having an affair while you were living together the Judge could take it under consideration when he splits your debts and assetts, but most times the court does not care.
~All should worship at the altar that is I~