[ZenGum]

Gaining sexual favours by deception is a crime, at least here.

There was a great case ni the papers a few years back, some office drone stuck out with a secretary, but spent two years convincing her - through fake personas and such - that he was an Australian secret agent, and that it was in the national interest for her to assist him, and this developed into personal support and eventually sex. I obviously didn't care enough to remember the outcome, though, I was just so amazed that (a) someone would go to that much effort for one particular lay and (b) the woman fell for it - if she really did, and wasn't just playing along.
I can see some genuine needs for this law - not of the 'you swore you'd marry me' deception but the 'Bill? Not Jim, the other twin/guy in mask etc?' sort of thing.