Well, let's look at the internal evidence. Lady M. has borne at least one child and nursed it (I, vii, 54). But Macbeth "has no children" (IV, iii, 216). That means either that their child has died. or that she had issue from a previous marriage. This last has the advantage of agreeing with what little we know about the historical Gruoch (see the Wikipedia entry under
Gruoch of Scotland) Which way you decide this question will bear on any insights you have into the relationship, whether you feel, for instance, that the loss of a child might have estranged them from each other or brought them closer together; or, if their marriage has been childless, whether their failure to produce an heir has been a problem for one or both of them.
There are not many happily married couples in Shakespeare, but Mac & Lady come reasonably close when we first meet them. Macbeth uses various endearments toward her ("My dearest love," "my dearest partner of greatness," dearest chuck"
, and they always seem to regard the plan to achieve the kingship as a joint project (again, the historical Gruoch's blood ties to the royal line were probably stronger than Macbeth's own). But the quickness with which she resorts to belittling, sarcastic language when he gets cold feet (I, vii, 35ff.) may point at some strains in the relationship even before the murder. Such insulting language continues in the scene after the murder ("Infirm of purpose," 'I shame/To wear a heart so white." Afterwards, of course, they appear to drift further and further apart, each into his/her private madness, and by the time of her death they appear totally isolated from one another. His speech coming to terms with her suicide (as it appears to be), "Tomorrow and tomorrow," seems nearly devoid of any personal feeling. It occurred to me while playing Macbeth some years ago that perhaps the reason neither can apparently get a good night's sleep following the murder-- surely a factor in their unraveling-- is that the marriage bed has itself been polluted. If the King comes to visit, you will of course give him the best room in the house-- the master bedroom. So they have murdered Duncan in their own bed.
Dramatically, of course, it seems to me that focusing on whatever positives exist in their earlier relationship (affection, respect, sexual attraction, common goals) make the slow death of their marriage a more pathetic part of the tragedy. So I think productions that create a real sense of love and caring between them give us a sharper sense of the terrible price both have to pay for their ambition and their crimes.