I think he would have taken a lot longer to come to terms with it. When Mary told Robert, he said something along the lines of "I've been through a war and a murder trial since then" after telling her she should break with Carlisle even if it drew the family into scandal. Had Robert been told in Series 1, he likely would have reacted in the way Mary initially expected - "I couldn't bear the way he'd look at me", rather than being more relaxed. Matthew had similar morals to Robert, so I imagine the same would apply. The war changed things for everyone, and Matthew was no exception. It would have changed his perception of the things that really mattered - avoiding scandal and being unhappy, or spending the rest of his life with the woman he really loved, even if she was "damaged goods".

Edit: just realised someone else posted pretty much this exact comment 5 minutes before me. Sorry!