The Coronation Street actress has allegedly put her wedding plans on hold after Lomas was accused of kissing care worker Katie Moores.
However, sources say that Marsh is willing to give the Hollyoaks star another chance and has "decided to believe" his protests about the apparent affair.
"Kym's very upset and angry, but Jamie's told her he was drunk and this girl manipulated him into this situation," a friend of the couple told Heat.
"Kym gets really low when things like this happen, but Kym has decided to believe him - she's so in love with him and of course they have a daughter [Polly] together."
It has been reported that Marsh fears Lomas will be hit with more infidelity claims as "girls try to cash in" on the story.
The couple have yet to publicly comment on the allegations.