Dear, you don't have to worry. You can draw whatever you want. You must not let people bring you down just because of what you enjoy drawing.
You can leave a warning and set the drawing as mature. If people complain them, ignore them, because they're being stupid and immature for not reading your warning. Draw whatever you want, and never let anyone overshadow you!

You could make a side account for mature content. Setting paintings for mature should help, assuming your friends hate all mature paintings and have opted not to see mature content. If they still harass you then obviously they have double standards and are hypocrites because they like others mature art. Can't think of any better solution though, so I hope this helps.

Harmon, you've crossed the god*mn line. You used to be cool, but now you've just been a full out d0uchebag. I know you're attempting to help, but you are not executing it well.

Potato, I will e-mail you as soon as I get my laptop back.

I've seen your gallery and see that you have taken time with your art and it's not 'mature', meaning you do want to draw those things because you put effort into them. Therefore, what you're really saying is that you want to ALSO draw other art. You certainly don't have to leave to do that. You can upload using the setting 'Mature' and that way only those who have chosen to view mature content will see them. You can create a side account and not link to it so it's not associated with your main account. You can also do what I do and put what seems too controversial for here on Deviantart, since it's a more mature art community. Whatever you decide, I see no reason for you to leave your friends here. Will people get upset? Of course. Even best friends can fight. Everyone has different interests, different personalities, and has a whole different set of issues that are going on in their lives. Sometimes we let our feelings come out wrong and hurt each other, but just forgive and move on.