Joe has faith in both Shane and his wife. Despite their love for one another, they are both loyal and good people and neither would disrespect him so much as to be unfaithful. He knows that if anything does happen between the two, they would tell him first and be completely honest. At the end of the day he knows that Marian is also extremely devoted to Bob and would never break up the family. In addition to his trust in Shane and his wife, he also has to admit that objectively, Shane is a greater man. It is not a surprise that Marian should love him, and rather than being a shock, it is completely understandable to him.
In many ways, Marian's feelings for Shane are very similar to Joe's feelings for Shane. Both Joe and Marian love Shane as a person—they both respect him and they both appreciate what he brings to their family and their farm. Both acknowledge that he is a wonderful man and feel safe having him around. Similarly, Marian is a wonderful wife, and it does not surprise Joe that Shane appreciates that. Shane's past with women is a subject never mentioned in the book—no one knows if he has ever been married or been in love. Regardless, he enjoys Marian both for her role as wife and mother, but also as a person. Between the three of them there is a mutual appreciation for one another that all of them understand and to which none of them object.