Niddah 15 - The Wife is Always Pure
All women are presumed to be pure for their husbands: a wife may cohabit with her husband without performing an examination. The checks that were learned about before were only instituted for women involved in preparation of ritually pure foods, in the time of the Temple and for some time afterward. Furthermore, the husband does not have to ask his wife if she is a niddah . Those man who come home from a journey can also presume that their wives are pure.
What was the need to teach the second part of the ruling, for men who come from a journey? You might have thought that the above ruling only applies when the husband is in town and can return home any time, and that's why we assume that the wife already performed any examination she might need to do, but, however, when he is out of town, she may not be so careful - the teacher had to tell us that the rule applies in both situations.
In explaining this rule, Rav Huna said that it only applies if the time for her fixed period has not arrived - because Rav Huna consider the needs to be more careful on this day as a Torah requriement given at Sinai. However, Rabbah bar bar Chanah said that when the time for her fixed period arrived, she is still permitted to her husband, because he considers this need as only instituted by the Sages, and here it is unknown if the examination was already performed.
Art: Ferdinand Bol - Portrait of a Husband and Wife 1654