People are free to talk about anything Zatanna related in this thread.
I shall refrain from judging whether something is sexist or not.
In that scene, John is set up to be the bad boy boyfriend, John Zatara is the over protective father trying to protect his girl from his clutches and Zee is the 'good girl' who is curious and drawn to his bad boy charm. Hence why she kisses him even after that remark about her body. TVTropes even has a page on this particular trope 'All Girls Want Bad Boys' ((
https://tvtropes.org/pmwiki/pmwiki.p...rlsWantBadBoys). It should be noted that John is also a conman so he was likely counting on Zatara's protective instinct and Zee's innate desire to help out so they will join in the seance which had a strong chance killing any of the participants (and it did!). The whole issue was him deceiving others into joining the seance, no reason to assume he wasn't doing the same here.
Zee's comment could be read any number of number of ways. It implied that the tantric studies and their relationship happened a long time ago when Zee was still in her fishnet costume. This implies that their relationship happened before she joined the Justice League (she was wearing her mothers costume when she joined and later switched to the Perez designed bug head costume). She had been with the League for almost 2 decades in real time by the time she met John Constantine so it's feasible for her to have forgotten their encounter. The other possibility is that she's had a few partners since her encounter with John Consantine so she genuinely didn't remember which would also be in character for her (which is not a bad thing either).
As for Reign In Hell, it's been a minute since I read it. I don't recall it affecting the wider DCU much when it came out. Its referenced in the first arc of Zatanna's 2011 series by Paul Dini but I don't think you will be lost if you hadn't read it.