I have a diamond frame Opafiets at our house and step-thru Omafiets at our cabin and flat in Amsterdam. I like both but have occasionally wished I had a step-thru at home.

In Europe almost all women ride step-thru bikes and an increasing number of men of all ages do especially in The Netherlands, Sweden, and Italy. In The Netherlands over 60% of non-racing bikes sold are now step-thru which would indicate that about 25% of men ride them. The Workcycles Gr8 and Fr8, two of the most popular new bikes in The Netherlands are not even offered in a diamond frame. WorkCycles Gr8 (as city bike)

I believe ALL rideshare bikes world-wide are step-thru as are most rental bikes. Cargo bikes like Bakfiets are nearly always step-thru.

For racing a diamond frame is still best but for daily transportation I don't think anyone would be able to tell the difference in a well made step-thru Omafiets and a diamand frame. 100 years ago it was a very different story but advances in metallurgy have made the straight top bar largely unnecessary except for fashion or carrying extremely heavy loads.

Oh, some people looking 'girly': https://www.flickr.com/photos/mellbin/

I'm pretty OK with girly. :-)