An observant reader was pissed off enough at Thesaurus.com to tip us off to this, and we share her rage. If you search for synonyms for the word "weaker" two main entries come up: Female and lady. [Thesaurus]
As someone who uses this thesaurus on a pretty regular basis and is familiar with the weirdnesses of its resutls, I can say that I think this is just a strange search thing, not sexism.
I think what happened here is that the thesaurus found the word "weaker" in the "synonyms" list for female and lady: "weaker sex". So female and lady popped up for the search. Women and lady are not actually being reported as direct synonyms for weaker, it's just that weaker is part of a phrase that comes up in their synonym lists. That's just how this little word searcher works. For better or for worse.
Not that I approve of the phrase "weaker sex". But it is an actual phrase. Alas.