I tested by performing only acts that had the dom tag once I'd gotten petting unlocked. After close to 50 days of testing, the slave's description still says, "...acts fairly timid and expects you to take intiative and ready to serve, rather than being served." That line also seems to show up after doing a few of any sex act, even those without either dom or sub tag, but it doesn't show up until a little while after starting sex.
Glancing at the code you pointed me towards and sexuals.gd, I can't see any obvious reason why that should happen. It really doesn't look like it's working properly. Something is causing slave.dom to drop when having sex without the sub tag, that's the only possible way the line ("...acts fairly timid ...") can show up. It appears as soon as that value drops below 40, and it shouldn't be changing with sex acts that don't have the sub tag.
EDIT: Two neutral and one dominant sex act will result in slave.dom dropping below 40. Three neutral acts will result in slave.dom dropping below 40. Two neutral acts and TWO sub acts is what it takes for slave.dom to drop below 40. For some reason, submissive acts result in less of a slave.dom reduction than neutral or dominant actions.