That seems like a great idea, Joe! Some possible explanations:
• The boyfriend is gay (or bi), loves kids (nothing weird or illegal), likes being called "daddy", but enjoys his "wild and carefree" lifestyle with other men.
• The boyfriend is some kind of celebrity, and cannot risk being associated with a "commoner", but he dearly loves the woman and her children because they give him the kind of "normal" life he has always dreamed of.
• The boyfriend is the late husband's brother, a paroled ex-con trying to get his life back together, but his former associates will do anything to drag him back into a life of crime.
• The man is Thomas Jefferson, and even though his mistress has birthed his children, custom and law dictate that she live in separate quarters because she is also his (most cherished) slave.