All of the males in Jefferson’s lineage had testicles, as far as I know. It is pure speculation to assume that it was Thomas. Did Thomas ever write affectionately about Sally? If not, it is dumb, disgusting conjecture. If he did, it still proves NOTHING.
It would have been socially unacceptable for a slave master to write affectionately about slave woman, other than maybe fond recollections of his wet nurse or the slave woman that raised him as a child.
The records at Monticello show that none of the other Jefferson related males were at Monticello when Hemings became pregnant. The records do show that TJ was at Monticello each and every time that Hemings got pregnant.
Plus none of the other Jefferson male relatives were in France when Hemings became pregnant the first time, TJ was in France and living in the same household with her at the time. Not proof admissible in a court of law, but interesting circumstances none the less.