So, with a little experimenting, I found that you can insert the slime core skill "reshape high" into another species, and it seems to work ok. I don't know if this is ultimately a bad idea, but it CAN be done.
Just insert this line:
"reshape_high":{"id":"reshape_high","has":true,"accumulation":{"skill_points":-2,"banked_skill_points":2}}
into your save file. I don't know if the order of where it goes matters, I put it adter the line:
"skills":{"version":3,"store":{"inspect":{"id":"inspect","has":true,"accumulation":{}}
Make sure you add a comma at either the end or the beginning of the inserted line, preserving syntax usually matters.
What this means, is that I can make a human character a futa, and it looks like sex scenes only check if you have a penis or vagina for the options. If you have both, they work.
Don't know how this might affect pregnancy, if at all though.