All right, so let's say you have this spacefaring race. They've been expanding, getting themselves into new environments, and having babies on completely new planets. The only problem is that some of these babies don't develop too well in these new environments; some are stillborn, some deformed, and some are just straight up missing things. One of these things is the Parthen version of a uterus.
For the latter deformed babies, the future looks pretty grim; without the possibility of bearing children, they'll look to other ways of attaining lasting happiness and satisfaction. Some may become doctors and biologists, and in their hi-tech world, they may seek answers as to how to solve their infertility.
Finally, after decades of research, a solution is devised: they can't repair the uterus, and it's too late to clone and insert a new one, but what they can do is hijack the uterus of a surrogate mother, splicing DNA from both parties to create a hybrid baby. 1008
And since the baby is getting genetic material from both mothers, it's got a fifty-fifty chance of having the same deformity, and needing to use the same process to have its own children.
I think you said somewhere that some of your Parthans already couldn't reproduce on their own; this solution could work for them too.
What I'm imagining is some sort of cyborg-genitals (never thought I'd say that), but given an advanced enough species I guess it's possible that they could just use some other parts of Parthan anatomy to get the job done. Perhaps the baby would get all of the 'father' DNA, instead of a split; that would make more sense to me, but may not work in your story. Also, with more of a biological solution, there could be more drastic physical changes due to hormones, leading to a more 'masculine' appearance (rather than a woman with a baby-making attachment strapped on).
After a while, this solution might not be the best one (that is, the original problem may have been solved in a more satisfactory fashion), but by then there would be enough 'men' around to sustain a viable population. Plus, by then the Parthans might have found more uses for them, such as the ones other answerers have mentioned.
What I'd like to stress with this answer is that it sounds quick and dirty, but that's exactly why it makes sense: people will go to great lengths to ensure the survival of their genes. The solution may not be the best, but given the direness of the problem, some concessions can be made. 1008