I would not worry about the friends with benefits thing if you are merely after sex.
Bottom line, most men now expect sex by the third date at most - in fact, plenty of male blogs now tell men to expect sex by the second date and loads recommend the first date.
So if you want sex you are going to get sex.
It is a sorry state of affairs for those men and women who want something more loving, committed, etc, but on-line dating is now a numbers game and the male blogs basically tell all men - even the nicest of men - not to hang around and do the long-winded courting thing as it is a considered a waste of time.
You want friends with benefits - you will have plenty of that. If you want Brad Pitt AND FWB then get to the back of a very, very long queue of ladies who are living in cloud cuckoo land.
You are right about "casual" - men see a woman who puts that down as being someone just looking for sex.
It is rubbish that "it depends" is not an option on any of these sites as all successful relationships should grow organically from friendship to something more. But it seems in the internet world you either just want to be friends, or just casually date, or want to get married immediately.
Be careful of any profiles without photos of themselves - usually the marrieds, both men and women, use this tactic.