If that was a deal breaker for the other party, yes.
I mean, how hard is to get? Anybody can deny anybody else sex for whatever arbitrary reason they fucking desire.
Maybe some people don't mind having sex with married partners, but some one will mind, and they are in their right to say "no i don't want to have sex with you because you are married", if they are lied to and someone says they're single, that's rape by deception.
If this goes through that's for the court to decide, but if the person was sincere about their belief (im assuming he wasn't, but this thread is full of what ifs, so let's pretend he was) why wouldn't it count?