The incest stress chance "compare" var calculation on line 611 in newsexsystem.gd really needs to be looked at.
I don't know exactly what about it is problematic (maybe globals.fetishopinion.find() isn't returning the right value?), but I do know that until I commented it out, and just set compare to a flat 500, even slaves with maxed incest fetishes would end up with 70+ stress after a single session with a relative (and that's after the reductions from orgasms). I didn't change anything else in the file, so clearly that was the culprit.
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I'll check it out, thanks for letting me know!
Edit: The previous value was incapable of reaching the 500 value. I re-evaluated the formula and it should be far more possible to reach that now. Mindblowing will guarantee no Stress on its own, but it is now calculating the Fetish, Beauty, and Lust more accurately. Thanks for bringing that to my attention! Issue resolved in v.9