I would love to give this a try - does that make me evil? 

A helluva lot less evil than the 'unfaithful git' who must have made his partner feel so worthless,  unhappy and ultimately suspicious, that she feels the need to employ 'another woman' to find out the truth, although I'm sure, in many cases, she already knows it. Just needs the evidence!

However, as my mind and words have unfolded on here, i've only just thought properly about the 'honey trap' job.
What a bloody dreadful thing it would be, in a way, to have to report back to a woman, that yes, she was absolutely spot on with her theory. I wouldn't get much satisfaction from seeing her world fall apart...
I'm sure there are many situations were the woman has been wrong and that'd be fine, taking the  money then.
Yes, there's a small chance the honey-trapper may end up on my doorstep. But unlike having an affair and all the time and deceipt that would entail... we ship 'em in and ship 'em out pretty damn quick!

basically if money is exchanged for sex it's against the law. It's just hard to prove what happened. And the clients paying for our time what happens in that time is consented between adults.

You make a good point.
But it's not against the law.