Now the first argument I always hear is that one line in Reckoning: I used to wonder what you looked like. Personally I shove this in the same boat as Callisto's family (mother, father, sister in one ep, Mother and sister in another, mother in yet another) and Xena's brothers (started as 3 ended as 2). Just another little thing that the writers ignored in later seasons. But other fans have been known to take this line and use it more as proof that they never met face to face. I could also see maybe an appearance change type of thing. Ares is a God and we know from other eps (Ties that Bind and Judgement Day in Herc) that he disguises himself. So maybe Xena was curious if he would use a different guise with her when she changed sides.
I have also heard the argument that he just watched her like a creepy stalker. However we know that he gifted her with the Chakram (sometime after Corinth and before Cirra) and taught her a few fighting moves (thank you Path of Vengeance for that one). Then there are lines from Livia/Eve eps on how Ares seduced Eve with promises of power the same way he seduced Xena herself. We also have Ten Little Warlords when Ares comments on Xena's touch still being the same no matter what body she's in.
Then we have the flashbacks from Armeggedon Now which actually show Ares with Xena and there were some scenes cut that very much showed a rather physical side to their relationship. Some say that sense Herc wasn't born the whole future was changed and thus these were not true flashbacks but what Xena's past was without Herc ever being born (I hope that made some sense). However, first off Herc wasn't involved with Xena at that point. He was involved with Ares off and on, but I think this time frame would have been fairly empty for the two based on the age of Herc's children when they died/the original action pack and Callisto's age. Also Callisto was able to know exactly where her family and younger self were and she knew what was going on and how they would die even though her father made a deal thus showing her past and the battle wasn't changed. And Ioloas was able to use his knowledge of Xena and her past (deflecting the Chakram, Solon).
What does everyone else think? What did I miss one way or the other?