If by "seeing" you mean having a sexual realtionship, then my advice is to document it, and then file for divorce on the basis of adultery.
However, if what you meanby "seeing" is just firenship, well, then it is Okj as long as you are included if you so choose.
Did he diclose his ->-bleeped-<- before you got marriage? or this is a recent development?
Jen61