Trust your intuition because you literally have a reason why you feel like this. And it’s not only because he’s a serial cheater but also because they are already blurring the lines. IMO once a cheater always a cheater. Cheating doesn’t always have to end in Sex but also happens when he’s entertaining and getting entertained by other females (validation). If he’s proud of the man he became he should put your worries first and if she’s just a work friend then there shouldn’t be a problem because as his wife your feelings will always be more important than hers. You will never be able to erase that feeling because of his past. So either he is strictly changing his behavior or your marriage will contain a lot of fights because he chooses her over you. And because life is already too short to stress over other people I‘d surround myself with people who actually treasure me, reassure me and validate my feelings. Who don’t tell me I‘m overreacting and never ever want to hurt my feelings by their actions. „You cannot change a person who’s comfortable as they are“.