Re: ... underpants "were considered a symbol of male dominance and power"
Given the era and area the feminist angle is rather moot... Think old-fashioned catholic *medieval* style thinking not unlike the more extreme "religious" opinions regarding such things.
As Hollerith1 pointed out, the style of undies is suitable for the then current fasion of the hose, which included an item called the "codpiece" . ( insert Blackadder reference here for Comic Relief )
Aside from practical issues others have pointed out, men of station and breeding were *expected* to bulge, and the long-legged boxer-type underpants everyone else, including the women, wore simply do not do the job in combination with a hose.
Given that in that era clothing directly equated to Status, and given that the actual purpose of the underwear is to achieve something that due to a simple matter of anatomy is exclusively male ( unless you resort to a stout pair of socks (scarth and lithp opthional, thank you Sir Terry) , it's easy to see how a piece of underwear becomes a symbol of male dominance and power.
Mine's the one with the chainmail hood.