The Mishna is almost exclusively a legal code and like all codes displays little 'emotion' even when it describes emotionally wrenching moments. It calmly lays out the legal principle for the case at hand and it is the rare Mishna that tells stories, spells out moral lessons or even asks questions. It is case based with the occasional formulation of a legal principle that forms the bread and butter of the Mishna[1].
"How does he proceed?" (Sotah 7a). Sadly a woman has been caught alone with a man with whom she has been warned to avoid exactly such. If she admits to having an affair the (sole) result would be the couple getting a divorce allowing them to move on with rebuilding their now separate lives[2]. But if she denies that anything illicit occurred we cannot accept her word at face value being that there are grounds to suspect her claims of innocence may not be true, leading to the sad spectacle of the ordeal of the sotah
Fascinatingly the husband must accompany her to the Temple where the ordeal is carried out. He too must share some of the blame and embarrassment of this sad situation and can't just have the wife brought by others to endure her ordeal alone. It is the husband who, by telling his wife who she may or may not speak with, caused her to become a sotah in the first place. The accuser must face the accused.
Our sages well understood that suspicion of marital infidelity does not necessarily mean that the last thing the couple wants is to see each other. "And we send two Torah scholars to accompany them" (Sotah 7a). I might have thought that they were needed to prevent dissension as the couple argues as they travel to Jerusalem. Such would be in keeping with Yosef's admonition to his brothers after he identified himself and sent them to bring Yaakov. "Al tirgezu baderech" (Breisheet 45:24), do not argue on the way, he warned them, fearing they would now begin to blame each other for what had transpired. 
Such apparently did not worry our Sages, rather they were worried "lest he cohabit with her on the journey."  The couple may accuse each other of infidelity but human nature being what it is that does not mean they can be left alone - they may be intimate with each other[3]. And if such were to occur the Sotah ordeal would be rendered ineffective and useless. Rabbi Yehuda disagrees asserting "her husband is believed regarding her".  
This debate is rooted in psychological assessment. The prohibition to have relations with a sotah is "only" a regular negative command as opposed to the prohibition to have relations with one's menstruating wife, a nidah, which is a prohibition of karet, excision, similar to the penalty for eating on Yom Kippur or chametz on Pesach. 
Rabbi Yehuda argues that since a man may be alone with his wife who is a nidah despite the potential of karet, surely he can remain alone with his sotah wife (whom he soon may be divorcing) as the potential prohibition is on a much lower level. To this the Sages retort that it is precisely because the penalty is so great - the Rambam asserts that karet means we are cut off from the world to come (Teshuva 8:8) - that he can be trusted. People may sin but they (at least those who assert fidelity to the teaching of the Torah) generally will avoid the "big sins" that bring karet. But with the sotah only being a regular prohibition we fear they may sin together. 
The Gemara then quotes a parallel source where Rav Yossi advances the same kal vachomer, a fortiori, as Rav Yehuda which elicits a different response from the Sages. The reason a husband may be alone with his wife who is a nidah is because the state of nidah is only temporary - when she goes to the mikva in a few days they can have permissible relations. However the sotah is potentially forbidden permanently and hence there is greater fear that they will engage in relations. In such a situation, the Gemara concludes, we can apply the verse "stolen waters are sweeter" (Mishlei 9:17). It is that which we cannot have that we want most.
[1] Pirkei Avot is the primary exception where we focus on the foundation to the law, that of character development. Interestingly the very first Mishna in the Talmud (Brachot 1:1) begins with a question and continues with a story. We introduce these features to highlight their overall importance even if practically speaking we will focus on the law itself.
[2] Being that it was the wife's affair that caused the break-up of the marriage she would lose the monetary benefits spelled out in her ketuba. When it is the husband who is at fault for the breakdown of the marriage the woman could demand a divorce along with her ketuba payment. (Of course in most cases it is not possible to assign blame to one side only when a marriage ends - adultery being one of the exceptions. In such cases the ketuba must be paid.)
[3]Once a woman is declared a Sotah the husband and wife are not allowed to be intimate - until such time as the ordeal is over.