He publicly opposed slavery and tried very hard to get it eliminated in the Constitution. The South opposed him and threatened to not support the new union without slavery. With no support from the South we were doomed to lose to the British.
Also, it depends on how you define many and I'm not even sure that Sally was not a willing participant.
"
Unlike countless enslaved women, Sally Hemings was able to negotiate with her owner. In Paris, where she was free, the 16-year-old agreed to return to enslavement at Monticello in exchange for “extraordinary privileges” for herself and freedom for her unborn children. Over the next 32 years Hemings raised four children—Beverly, Harriet, Madison, and Eston—and prepared them for their eventual emancipation. She did not negotiate for, or ever receive, legal freedom in Virginia."
https://www.monticello.org/sallyhemings/