Odds are good if you're married to someone of a different race, your in-laws are of a different race, and you know your kids are going to be mixed, that you're probably not racist.

Precisely.

I'm not sexist because my wife is a woman?

That does not work due to fundamental differences in how racism and sexism typically manifest.

Up until about the 70s or so, racists campaigned heavily against interracial marriage. Most racists likely still hold these views, though after 1967 they are no longer relevant so you don't hear about it very often.

I do not recall any sexist groups ever campaigning against heterosexual marriage. Now, sexists have campaigned against laws that would equalize the rights that people in marriages have.

Therefore "I am not sexist because I believe that my wife and I are equals and should be treated as equals by the law" would be an analogous statement.