5% might be a bit optimistic. Here's something for comparison:
Greater Seattle area population 3.5M
Average NFL crowd 67,000 (Sell out) - nearest rival 800 miles
Average MLS crowd 44,000 - nearest rival 160 miles
Average MLB crowd 20,000 - nearest rival 800 miles
All these crowds seem to have a far greater percentage made up of women/children/families than I remember at football games.
NFL being the most popular and selling out every game but I doubt they'd get 5% (175K) even if they had a stadium big enough. 1M did show up though for the parade after they won the Superbowl.
Every major league sports game is televised live and I often wonder if that helps the crowds and fuels the enthusiasm for the game rather than reduces the crowds as many think it would.