The ratio of children being born is roughly 50/50, though, as most other places in the world. Here in the US there isn't a lot of abortion based on gender yet (thank God!) so birth rates comes out pretty even.
What you could have, then, is people moving to the area for jobs. The military in particular is going to bring in a higher ratio of men than women, but they are probably not looking to do a ton of dating while in active duty... and they are moving in and out of the military bases around the area every year or two.
So the other major employers - MS, Amazon, Boeing: are they really bringing more men to the region than women? Maybe, but only a little bit I think. Anyone they bring in who is already married or in a committed relationship is likely bringing their partner / family too. And I know that with Boeing in particular a massive amount of the workforce is already from Seattle; they aren't trying to hire people from the other side of the country for mechanics positions, though some of that may happen in the upper echelons of engineers and managers.
So what I suspect is happening is a *small* influx of workers to the area, with a bit of a higher male:female ratio. There is also some movement of folks from around the greater metro area into the city as young working professionals, to be closer to their jobs. That may make an imbalance within the job hubs of Seattle, Everett, Redmond, and Bellevue... but what people are leaving out are the myriad of smaller cities around and between them. Do Kent, Renton, Auburn, Burien, Kirkland, Bothell, Woodinville, Lynnwood, Shoreline, Edmonds, Marysville, Federal Way, Fife, Puyallup and the many other towns all have more men than women as well? I doubt it. I bet if you look at the region as a whole you would have a 1.1:1 ratio or lower.