John Gill Commentary Romans 7:3

"So then if, while the husband liveth, she be joined to another man, she shall be called an adulteress: but if the husband die, she is free from the law, so that she is no adulteress, though she be joined to another man." — Romans 7:3 (ASV)

So then if while her husband lives
True indeed it is, that while her husband is alive, if

she be married to another man, she will be called an adulteress ;
she will be noted and accounted as such by everybody, except in the above mentioned cases:

but if her husband be dead ;
then there can be no exception to her marriage:

she is free from the law ;
of marriage, by which she was before bound:

so that she is no adulteress ;
nor will any consider her such; she is clear of any such imputation:

though she be married to another man ;
hence it appears that second marriages are lawful.