I think the "womanizer" discourse comes from Zack mentioning having "multiple girlfriends" in OG (JP). I have not seen the original JP script so I am not sure if that is a mistranslation or if the was actually written.
@Ryushikaze Do you have insight on this?
There was also the Maiden novel where Aerith calls out Zack's flirty ways. (This is not canon material but there are some people who still believe it to be)
If "multiple girlfriends" was a confirmed in OG (JP), it seems to be retconned because there is no mention of Zack having multiple girlfriends in the Ultimanias or other official FFVII media in the compilation. Ultimanias have mentioned how Aerith is special to Zack. The "girlfriend" that Zack mentions in his letter to his parents is implied to be Aerith. And in an interview, Nojima said that he did not intend for Zack to waver between Cissnei and Zack.
On a side note - I think womanizer is the incorrect term to describe Zack because he wasn't involved with multiple women at one time nor was it implied that he did anything sexual with them. I read his character as someone who is charming and playfully flirtatious. And when he met Aerith, he was only committed to her.