The Gemara explains that הִרְהוּרֵי עֲבֵירָה קָשׁוּ מֵעֲבֵירָה - thoughts of trangressions are worse than the transgression itself (Yoma 29a)
The Akeidat Yitzchak explains:
When the Talmud Sukkah 52, states that anyone who is greater than his fellow man also has greater urges than his fellow man, this may well mean that he who deliberates before committing a sin, may well find that eventually the impulses urging him to sin will overpower him. This is also the meaning of the Talmudic statement (Yuma 29) that "the deliberations leading to sin are worse than the sin itself." This is nothing else than saying "when the sin has become the result of careful deliberation, such sin is especially weighty."
So, Reuven did not anything but rearranging the couch/bed. However, it is important to find the reason why he did that action in the first place, and that brings us to the commentary of the Daat Zkenim:
The Torah treats the deed as if Reuben had committed incest with his aunt. Our author points to Job 31,12, [he could have used the entire sequence from verse 1 to there Ed.] as proof that harbouring incestuous thoughts may be considered as if one had carried out what one had fantasized about.
So, he did not actually sleep with Bilhah, as the Tur HaAroch explains:
Some commentators point to the wording of וישכב את בלהה instead of וישכב עם בלהה, as meaning that all Reuven had done was to put his bed next to that of Bilhah. [the expression את always denotes a lesser degree of intimacy than the expression עם. (compare Genesis 30,16,Samuel II 11,4; ) Ed.] Further proof that Reuven did not actually sleep with Bilhah, is provided by Yaakov himself in Genesis 49,4 when he describes Reuven’s indiscretion with the words: כי עלית משכבי אביך, אז חללת יצועי עלה, “for when you entered your father’s bedroom you desecrated him who ascended the bed that had been made for me.” All Reuven had been guilty of was to disarray the bedclothes, not to lie in them. He spoke about the person lying in the bed in the third person, instead of saying: “you lay in my bed.”
But it was his Yetzer Hara that got into him and made him move the bed. Instead of controlling himself not to listen to his Yetzer Hara, as we see with Yosef HaTzadik, he actually did and moved the couch. According to the talmudic principle of הִרְהוּרֵי עֲבֵירָה קָשׁוּ מֵעֲבֵירָה - he commited a sin.
