I've seen females get offended after finding out guys do that. personally, I see nothing wrong with it. the thing is a guy calls dibs and his friends understand he's into a certain woman/girl and they back off. whereas almost everytime I've told a female friend I was attracted to or liked a guy and I was planning to make a move on him she'd do so before I had a chance, even if it was a guy she made fun of before. which is why I no longer tell anyone
Then you must have some horrible female friends. I would be really angry if a friend did that. The only thing I don't like about dibs is that the opinion of the woman isn't really involved. What if she likes his friend more but one guy has dibs on her so he won't act out on her interest. I've had this before and it's pretty dumb. So I think it's just a normal thing to do to not go after your friend's crush if he/she told you he/she likes him/her, but go around with it mildly. So let you final decision to the girl/boy.
It's that catty nature that females have. I had a roommate in college that used to do that constantly. She'd hear that one of us was into a guy and 5 minutes later she's all over him. Spiteful little itch she was.