The planet Transsexual is flourishing. Over the past century, there have been fantastic developments in life extension, reversing the impact of aging, and general fortification of the body. True immortality hasn't been reached yet, but scientists are hopeful that it's within reach. After that, the only medical advancements needed will be in granting this boon to other races as well. A task for the current generation's children, or their children's children…
It transpires there's just one small problem with that.
Nobody is in agreement on what, exactly, caused the infertility crisis. Which of the several immortality treatments is responsible for women failing to conceive or carry to term. It might not even be one of the medical advances, might just be a metaphorical time bomb left by the Unpleasant Times Better Forgotten Entirely of two centuries ago finally gone off. Or a combination of any of these factors or others not yet considered, of course.
There's hope when it's found that some people's reproductive systems have been unaffected — hope that quickly diminishes when genetic testing shows that everyone who carries the gene that grants this reproductive robustness is closely related enough that it would be a bad idea for them to have children with each other, at least after a generation or two.
Many doctors and scientists are researching gene therapy, splicing, things to allow the less-fertile population to piggyback off of the luckier ones. The Transsexual Military has decided to search for fresh genes a little further afield.