Im inclined to say boy but I have a few reasons why I dont feel that is correct all the time.
#1 In the 2003 Series Ed and Als Mother is made a Homunculus as well. Now in both series you see her as a Hard working single mother going around and doing house work raising two boys and keeping food on the table.
Her Homunculus in the Series is Sloth... SO her Life before the Change Didnt make her change make much sense. She didnt seem very slothy and I know from my mother whos a single mother raising two boys that Us males can be quite a handful when we want to be.
#2 In Brotherhood Envy's True form is that of a Leech with legs. So theres very little Sign there to show that Envy is male not only that but Leechs are all Hermaphrodites (I.E. something or someone that Holds all the reproductive organs of both males and females) Therefore the Saying that Envy is a boy is up for Debate.
#3 Envy Refers to themself as a Male. but if you look at the timeperiod of which Brotherhood and 2003 are placed in both are Technologically Placed in the time before the Burning of London or around said Time period (Sherlock Holmes era as well). Basically Males are Seen as the more In control and the More incharge of the two genders where as Females are the Stay at home and in the kitchen.
THeres also the case of Automail which reminds me of Steampunk which is Reinessance era so that Leads more so to the view that Males are the more in charge gender.
That being Said Envy would Of course use Male references due to the time and society that They live in. Not only that but Envy Has been shown as having Many souls due to a philosophers stone and actually Displays them moreso than the other Hommunculai leading to the belief that unlike other hommunculai that have Chosen a Dominate Gender and or personality Envy doesnt which works for Them on the Shapechanging. No Man can play a woman 100% and no woman can play a Man 100% but both can Play the other given the circumstances to an amazing degree of believability.
Therefore I shall Classify Envy as 100% Grade A He/she