I'm not buying your logic:
She would have the bf for his money. If she needed to have sex with him to keep him around, she would. After all, she had plenty of sex with me before we got married.

Then why doesn't she have sex with her husband to keep him around?
Because she needs more money to do that?

Why doesn't she look for a new man with more money?
Because "..neither of us is willing to do the work to make any other relationship successful"???

Like I said before, it isn't about money and sex.

Here's another mind game for the two of you. Have a RH discussion about your problems other than $$ and SF. Those are off limits. Pretend they don't exist. After all, both of you say they are insignificant amounts anyway. See what other issues you come up with.