It does depend on the marriage.
Some women are perfectly trusting of their husbands, and vice versa - a female friend of mine regularly went on camping and caravanning holidays with her male best friend and her busband's blessing.
Some men however, are not immune to the intimacy of a platonic frienship, and can see signs that are not there, because they wish they were. This can lead to complications, and it is usually the man who makes a move, and spoils the friendship - it's a bell you cannot un-ring.
That said, plenty of people enjoy close platonic relationships with members of the opposite sex - it does come todn to the individuals involved.