Is there any objective downside to riding a women's bike as a man? I'm shopping around for a cheap, cruiser-style bike for my commute. As an example, AroundtheBlock from sixthreezero seems like it would fit my needs. However, the colors of their men's bike are really boring. The women's version has better colors, and I'm wondering, why don't I just get that one?

I had always heard that the primary difference with women's bikes is that the top tube is lower to accommodate skirts, and otherwise there isn't really a difference. While I don't wear skirts and don't have trouble stepping over a men's bike, I don't see how it would hurt either. Presumably it alters the mechanical properties of the frame in some way, but would that be noticeable enough for my intended use?

My commute is 2.5 mi long and completely flat. I am also an experienced rider and have no significant joint pain or problems associated with biking. I'm an adult man of average height and weight.
If the bike fits, ride it.  

The average woman has different dimensions to the average man, so the make bikes with different dimensions to better suit the avenge woman and call it an womans bike (and, as you have noticed, they paint them a different co lour). Problem with all these averages is very few people (if any) are average and the range of sizes of men is significantly more than the difference in size of the average man and average woman.  

Probably the biggest difference is an easy one to change, the saddle. You may want to go for a narrower saddle than the bike comes with.