Guess this is like wondering about the sound a tree makes if no one hears it, but, if a lady you have seen before arrives at your place where money lays on the counter and then you fk and then she leaves with the money....has a crime possibly been committed? Assuming you're not charging her with theft. ( If you did charge her with theft, does she have a defense?)

Yes, you paid her for sex.  They still call that prostitution and prostitution is still illegal.

You got it.  The theft charges is obvious, especially when she makes a deal with the prosecutor no misdemeanor vice charge to get you on a couple felonies.

With some ladies I know well, I've paid her outside of the "date". Thus, if a bust ever occurred, LE cannot easily find any evidence of payment.

In the first situation.  It's been the custom so long that leaving money on a counter for a lady to take after sex has been in movies before Jane Fonda was acting.  No problem for a prosecutor to make a prostitution case out of it.   Whoops.  One problem.  A witness every juror will believe.

Second situation,  what her defense is "I was committing another crime?"  My money is they won't believe the theft anyway, but they'd believe her confession.  Now if you went for both.