system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why do bikes for men have that top tube in the frame?

I understand why it would be easier for women not to have that high bar in the bike frame as it would not work with long skirts and dresses. But as the... area, so to say, is quite sensitive for men, why does their bike frame have a top tube?
gpt
A well fit men's bike should never really get in the way of their balls when standing over it. Stunt bikes tend to be very short, and mountain bikes need to be as strong as possible, which a top bar is. BMX bikes actually are made with a slanted top bar, specifically to avoid "racking" your nuts.

The lower top bar (or missing top bar) was put in place mostly for women's fashion. Originally it allowed biking while wearing a dress. Later, tight pants. The low bar allows mounting without having to lift your leg really high. This actually makes women's bikes really popular with older women, who are (at a younger age than men) more likely to have hip mobility issues. 

The downside of a lower (or missing) top bar is that you lose a lot of the strength a traditional top bar adds to the bike. You're left with either a less resilient bike, a heavier bike, or a much more expensive build. When women still had to wear dresses, they were also expected to ride in a much more reserved manner, so the weaker frame wouldn't have been as much of an issue.  

You can still find modern bikes made with this lower or missing top bar, but if you look at top of the line women's road or mountain bikes, you'll find they look very much like a men's bikes. Giant's women's road bikes actually have a slight nod to the dropped top bar, with a slightly downward slanting top bar - which is more aesthetic than functional. Most of the changes have to do with the parts on the bike (special saddles, smaller gap between handle and brake or handle and shifters, and the frame usually accounts for a shorter torso. Generally, more ergonomic choices (though some cosmentic) and less... well sexist. 

For a casual bike to ride around town, where weight doesn't matter, a dropped or non existent top tube is going to be easier to get on and off, and more easily set to a variety of riders. In fact, many bike share companies choose this set up for that reason. However, once something is labeled as "women's" it is really hard to convince men to get over that trend en masse.