I happen to think it's morally wrong, but it also tends to turn out poorly eventually. No personal experience, but having watched this happen to girls I knew in college, I think in "relationships" like that, the guys tend to come out on top. The girls usually agree to that status because they like the guy or are crushing on him, and for some stupid reason think that having sex with the guy will make him want to turn her into his girlfriend. Pretty soon, the guy finds a girl that actually attracts him and makes her his girlfriend, and the "friend with benefits" ends up super messed up emotionally. Just to make it clear, I'm not blaming the guy, because it is always the girl's own fault for agreeing to a relationship that has only an infinitesimally small chance of working out the way she is hoping it will.
I don't think the "one will inevitably want more" argument is infantilizing adults. It's the honest truth, and the one who wants more is usually the woman. Assuming she hasn't had sex with a whole bunch of guys before, when she has sex, she creates bonding hormones with the man. Those bonding hormones make her want to have a relationship with the man, and the longer she has sex with the same guy, the stronger that desire gets. A woman can go into a friends with benefits situation with no desire for anything other than the occasional sleepover, and end up wanting the guy to marry her. Even if she tells the guy that, it helps absolutely nothing if he doesn't want to marry her. How can you "come to a consensus" if the guy doesn't have feelings for her and wants to continue on as things are and the woman has feelings for him and wants to get married?