To be most correct, it depends if you have taken five or four lay vows.

To be more careful then if you want to take all five precepts, then [best] to not engage in sex with that person. But if you haven’t taken the sexual misconduct vow, then it’s OK to [have sexual contact with that person.]

If you haven’t taken the five vows yet, then just take four vows, so that means not taking the sexual misconduct vow. So for the time being, you could do like that.

This is an unclear area, as they are legally married, but then many people have no connection at all with their husband or wife, although they are still legally married. So it’s a little unclear if it refers to legally or according to reality. This aspect of the question needs to be checked—if the vow refers to the legal issue (the aspect of being married) or to reality.

I think maybe it should relate more to reality, more to the connection, rather than to the legalities, but of course you can bring up the legal issue as an argument.

With much love and prayers ...