It's true that there were a pair of models of FSM back in ye olde Rogue Trader days (Female Warrior Gabs and Female Warrior Jayne), but they weren't in the first batch. The first space marine model was the metal LE2 Imperial Space Marine in '85 (or possibly '86), followed by the RTB01 plastic kit in Sept '87 which came out at the same time as Warhammer 40k: Rogue Trader (aka 1st edition). Gabs and Jayne were metal models ordered individually from a miscellaneous "Adventurers" release in Spring '88.
Personally I don't think they sold like shit because they were women, either. I think they sold like shit because they look like complete shit. Even the best paint jobs on those two models still look awful.