Sonja is wrong with the "Only one who can defeat me" business. I take the view that it's sort of a rationaliztion/denial defense mechanism. I don't think she was ever contacted by a goddess. I think her vow is a way to distance herself from man and to avoid getting hurt.
I like her encounters with Conan because she drives him up the wall. I don't think Conan even see her in a sexual way. In every encounter I've read about them, Sonja has either defeated or outfoxed the barbarian. I think Conan just wants to defeat her, he would probably walk away without claiming his "prize."
I go back and forth between whither Conan and Sonja are a OTP or should just be best buds and allies.