why not male protagonist with female enemies ?
Oh boy, there we go again...
I can offer you a certain explanation that I've given on quite a few occassions after quite extensive research of character reception for my own little projects and when I was helping two guys with their games.
1.) Like with a lot of games, hentai or not, players are primarily men and want to look at pleasant things. That's why people, given a choice, usually take female character anyway. It is a double-edged sword since if you make females ugly, people will linch you xD
2.) This is related to hentai games mostly - resources.
If you have one character to worry about in terms of design, it is much easier to make it hotter and not repetitive..since it is the only one and you can really put your effort into it. Other characters then can be so-so because the main powerhorse will be the PC.
Now, if you make a male PC, you have to not only make it somewhat appealing but also make a lot of other characters hot as well. You see the problem now? Instead of one resource heavy design and a dozen of so-so, now you have to make one so-so and a dozen of resource-heavy hotties.
You can ignore that, sure...but people will bash you for it. They will complain that XYZ is not hot enough or is disgusting or something. You need to be more creative with every enemy. When you do female character, you only need to nail it ONCE.