If Washington has a law that forbids women from exposing their breasts in public, that law can be changed. Similar laws in effect elsewhere have been changed. The Topfree Equal Rights Association changed the law in Canada.
A group called "Breasts Not Bombs" has fought to change California law on this subject. The "Topfree 10" fought the same battle in Florida. Washington may have a similar group. I agree it is a ridiculous law and it ought to be changed.
Personally, I agree that such laws are discriminatory. Unfortunately, the Washington Supreme Court disagrees with me. In Seattle v. Buchanan,90 Wash.2d 584, 584 P.2d 918 (1978), the Court upheld the convictions of five women who were sunbathing topless in the Arboretum. The women were charged under an old Seattle "lewd conduct" ordinance. The Court said that the law didn't violate equal protection because women's breasts are associated with sexual arousal whereas men's are not.
I don't know offhand what ordinance, if any, might apply in Sumner. Seattle's current ordinance makes exposure a crime only if it is likely to cause "affront or alarm." I doubt that you'd affront or alarm anyone if you mowed your lawn shirtless. Sounds like your neighbor is the one doing the affronting.