And using one social framework I concede that you are on solid ground. The system is designed to provide more benefit to people who;
a) live longer
b) have less lifetime earnings
c) have a spouse who made much more then themselves during their career and/or died.
Undoubtedly, the lawmakers who designed it this way knew that people who fit the above demographic are generally women.
Is this "sexist" using every potential definition? I think there are good social reasons to skew the benefit formula to the benefit of people meeting those criteria, even if the outcomes of that calculus benefit women disproportionately.
Strictly speaking, (with the exception of lifespan) we all theoretically have the opportunity to live our lives in a way that maximizes the SS benefit.
I guess what I'm saying is maybe it's sexist and still justified. I think widowers who live a long time in retirement after a working-class career deserve to be looked after too.
It's inarguable that SS outcomes are heavily skewed toward women, and although I have mixed feelings about the justice of it, I think it's entirely appropriate to educate people to its existence, and point out that "the patriarchy" uses this fact as a sales and marketing tool.
It is designed this way because men are socialized to take care of the women. That is the patriarchy.