I dont know if it has anything to do with it, but If i remember right mollies are technically brackish fish -- where did you obtain the males? did they die right after acclimation? if they did, did the pet shop keep them in brackish tanks or fresh water tanks? (heck, ive even seen them kept in salt water tanks with a slow enough acclimation) What about water conditions? Live Aquaria doesn't have a blooded molly, but the recommendations for a black molly are "Tank Conditions: 68-82°F; pH 7.0-7.8; KH 10-25".
sorry for your loss
-- lets get to the bottom of this, maybe it'll make it easier..