Sounds like you answered it right there. You weren't expecting a sexual conquest before the meeting. It led to sex, but that's not the result you were counting on before stepping through the door.
Mary's point, to me, seems to be that she very rarely if ever has the thought process before meeting with a man that they might get down and dirty, and that it's being implied here by men that they
do have that thought process, even where it isn't applicable or appropriate. That, even if it is inappropriate, men think it's fine to make those thoughts public and known to the intended subject of fantasy. That the expectation of sex during any private encounter with a woman is "reasonable" to prepare for, and that Mary finds this questionable and creepy.
Which I agree with, if my interpretation is correct. If you have a complicated history with a specific person, expectations can rear their head despite one's best intentions. But it gets a little, erm, interesting if this expectation is present with every interaction with a woman, be they friend, acquaintance, or a stranger that has yet to even be met. Along with the side sentiment in another thread about how it's seen as an objective failure to not have had sex yet in 2019, and how every rant or rave involving a woman ends with either "I didn't have sex with her.

" or "I'm going to try and have sex with her!

"