Honestly, we went to mixte's many years ago to avoid the oopses... Remember those old fashioned 27" French stepthru 'kool' bikes that even came with luggage rack eyelets. Never noticed them flexing; I'm small. My current one with a 1985 steel frame is stiff, rather light, very maneuverable and cruises right along even with the panniers and porteur. Odd enough not to attract quick snatchers too.
However as surfaces continue to decay and off road capabilities become more useful, we are now looking at those newer low straight tube 26" mountain bike frames for a more sturdy 'suv'. Are they 'ladies' bikes? Maybe 'step overs' is a better term? Yeah... "STEP OVERS". Wait, what was I thinking...
Hey though, maybe women's bikes are also thus less likely targeted for theft.?
Be well.