as salaamu alaykum wa rahmatullahi wa barakatuh,
In the Quran and in the teachings of the Prophet Muhammad (peace be upon him) - the two major sources of Islamic law - Muslims are commanded and encouraged to be honest and truthful in their dealings and to avoid deception. This is a general principle which applies to all relationships and transactions:
Deal not unjustly, and you shall not be dealt with unjustly. [Al Qur'an 2:279]
There is a very important point here that I would like to mention: There is a distinction that needs to be made between a marriage being 'technically valid' and it being one that a person will be rewarded for and that is considered acceptable, good, and within the moral and ethical guidelines of Islam. An action can be outwardly valid from an Islamic legal perspective, in the sense that it fulfills all the necessary conditions and requirements (two witnesses in the case of marriage, an offer and acceptance on the part of the bride and groom, an exchange of a gift, no time limit put on the marriage or similar restrictions, etc) BUT based on a number of other issues, including the intentions of the people involved, it may be something loved and accepted by Allah, or it may be something which He ultimately punishes the person for.
Another example would be a person who performs salah [ritual prayer], fulfilling all the arkaan and shurut [internal and external essential elements of the prayer], yet the entire time their mind is focused on something else entirely, or they are praying on land or in clothing that was acquired in ways that were not permissible. The prayer itself is technically valid, but how much closer to Allah did it really bring them? In truth it may be that they have actually accrued sin and further distanced themself from Allah all while performing a technically valid action.
In essence, what I am trying to say is that we should consider each action in light of both the letter AND the spirit of the law and a Muslim's actions should be reflective of both.
Allah knows best,