Even if a marriage record cannot be found - if the evidence suggests they were living as husband and wife (such as a family on a census record with man, woman and children) then they can be sealed. You will note that as part of the submission process - nowhere is there a place where you are asked to provide proof . . .
of course we should look for a marriage record in our research. But such a record may not always be extant.
Short Answer: 'Yes', provided that you have 'evidence' that they had a "Relationship"; they "Lived Together"; and/or, the Husband 'supported' the Family.
Whereas, for 'one night stands', where a Child was born from, the answer is 'No'.
Here is a "Knowledge Article in "FamilySearch":
Policy on sealing a deceased couple that were never married nor ever lived together
Where it states:
A deceased couple, on an exception basis, may be sealed by proxy if there is an indication that they lived together but were not legally married to each other during their lifetimes.
It is inappropriate to create a relationship for eternity when none existed in mortality. Avoid “matchmaking” when there is no indication of a couple having lived together. For example, deceased couples should not be sealed simply because they had an affair, dated, were friends in life, or had some relationship other than a spousal relationship.
Children who were born to parents who should not be sealed can instead be sealed to their grandparents who were married.
When recording the couple's information, do not enter a marriage date or place.
I hope this helps.