There are absolutely no visual distinctions between males and females despite the many rumors of "long or pointy fins" suggesting male. I've seen this theory fail many many times... I currently have a female Blue Dempsey that has longer/pointier fins than any standard male Dempsey I have ever seen...
If you do not wish to breed them, sexing them with certancy is unimportant. Just decide what you want it to be and say that's what it is...
If you do wish to breed there are many steps that you should be taking to make the nicest pair possible... which I won't try to summarize my opinions on in this thread
I wouldn't say extremely hard to mate but patience is definitely of a vertue.
Here is a pic of my breeding BG male and female. Visually I can not tell them apart. The amount of spangling is about the same. The only way I can tell is by there aggression.