Her baptism is a big deal to me as far as becoming a full fledged Christian; I am happy about it, but really feel ambivalent given that she is doing it while committing what Christians would consider a horrible sin.
I told her I'd like to attend and she said she would like that, but wasn't sure if I'd be comfortable given "the other people who will be there", namely, the OM.
Her baptism, like your wedding vows, means more to you than to her.
Her ongoing affair reduces this to (begging Monty Python's pardon) "a farcical aquatic ceremony".
After a brief dunking, your wife will be wetter and more outwardly sanctimonious in her delusions.
Go Dark. Work on Pigskin. Leave the moistened bint to discover the error of her own ways or not.