For "entertainment".
WARNING: possibly offensive material ahead - read at your own risk. (I've tried my best to keep the wording safe but sometimes it just doesn't work!)
Because if it is as you say, and the Parthans are "otherwise identical to humankind", it's likely that for them, using plastic/metal/wood/whatnot as entertainment is probably not enough to satisfy at least one part of the population.
Some mad Parthan scientist is bound to start engineering biological "toys" which could end up feeling a lot better and result in more advanced biological toys which(which eventually turn into males).
This could easily spread - the first biological toy (which from hereon shall be deemed "D1") was used by only that scientist - then her friends decided to borrow D1 for a party or something (Humans do like to party) which turned into research grants and papers being published about biological toys. D1 may have just been a lab experiment, but it quickly turns into D1.1, D1.2, D1.3, with more functions, shapes, sizes and colors.
Eventually, we have D2. D2 consists of what looks like a human hip + D1, and includes some autonomous piston action. Various fetishes and ideas turn into different functions which just so happen to require various organs/bone/muscle. Perhaps D4.3 involves abdomen muscles and thighs in order to provide better "angling". Perhaps D6.3 happened to require a ________ system to satisfy ____ fetish - you get the idea.
The cycle continues until eventually you have your first male, which only has one purpose - to lay there and look sexy and provide "entertainment".
It just so happens to turn out that these bodies that are laying there are taking up a lot of resources and doing nothing and using valuable space. So the Parthan group decides to put them to use, and finally give it sentience. Voila! Your first male!
TL;DR and for those who still don't understand what I'm referring to: Males can come about as a result of sexual needs - a need most humans love to satisfy (usually via sex)
Note: I saw answer(s) that focused on the validity and the possibility of the OP's scenario. I'm explicitly choosing to ignore all that, and actually answer the question posted (Which is actually "What other specific reason or cause might fuel a group of Parthans to make this decision to engineer males within their own species?", and not "Is this possible/likely to happen", and certainly not "How would this process work?")