Not knowing that particular case, but.....(as I am never one to keep my very opinionated opinion to myself!)
I honestly believe (so, stone me) that many men in particular are fairly, um, well, I don't know how to say this. Kind of dumb and trusting? I mean when they are romantically or want to be romantically involved. (which is what Lissie was saying)
I honestly believe that my husband specifically "slid" into an affair with some chick from work. Thinking he could and would "help" her with whatever her problem was. Shirley Glass actually says that "good Marriages" are more at risk than bad, because many believe that a good marriage is an innoculation against an affair.
And it ain't.
Yes, I'll post the link again. It's a fascinating book.