Some of my best relationships have been friends with benefits. As along as you are honest and open about it, get tested for STDs, and/or practice safe sex, and set boundaries and not distance yourself after and make it weird there is nothing wrong with it. And you're not a HO unless you got paid for it.
Who cares what other people think, if they are not paying your bills, and won't be there for in your times of need, their opinions don't count.
As far as telling them about each other. I would suggest it so they don't think you only want to be with just them and get hurt. I was dating 3 girls and they all knew about each other. No lies.
Because of the honesty I'm still friends with 2 of them. I was friends with teh 3rd but she is no longer a friend due to her lack of consideration and games.
True, we shouldn't be bothered by what other people think, but that always affects me somehow. Maybe I am too self conscious.