If you are talking law, lying to get someone to agree to something (sex, selling of a property or whatever) invalidates consent. Sorry to tell you, but there is an specific legal terms for this "vitiated consent". Sure, the lie is difficult to prove, but if it is proven, then consent is not valid and the consequence is that the sex obtained that way is rape.
The strong reaction you have to this, calling it "cunning bit of conning" just shows that you deem it acceptable to lie or manipulate women to get your way. But I'm willing to bet that when you are with your female relatives or women you like, you probably get offended if they suggest that women shouldn't trust men...