I think it's fair in some cases because not all artist feel comfortable making NSFW content, so I guess the only expedition for some people to draw NSFW only if they are paid to do it seems fair.

While I'm no artist and could just be talking out of my arse here, if it's for no other reason than "It lewd" then it just seems like a way to get a little extra dosh.
If it's because drawing a dong or a vagoo adds enough extra work to charge for it, then I'd be iffy on it. If it's ONLY genitals on show then the charge is pretty suspect, but if say, it were an explicit scene then yeah maybe I could get the artist's angle.

Honestly I've never come across an "Adult Free", but I can imagine it being contextual as all fuck xD

Up to them, not that I'd think it'd help them any and might actually hurt their sales.

If up to now they've refused to do anything but clean they might get away with it, but if they've been down and dirty the whole time I don't see it working for them.

YMMV as they say. (and if one artist says no to bare tits there are dozens of others that will say yes because they need/want the money.)

I think that's up to the individual. I'm not upset when I see there is a fee for NSFW.

depends on the amount really, I am willing to spend an extra $5 if need be but more than that and it gets a bit too rich for my tastes (purchaser, for it in small amounts, flat fee unless hyper/main focus of the piece)

You have to think of an artist as a shop/store
If you want to buy something exclusive to that artist then you have to expect different fees that come with that exclusive artist.
If that artist chooses to charge NSFW fees, then that's their choice.
You can't expect a flat rate for every single artist to come into FA trying to sell art. I think artists have a right to charge according to their artistic skill. I am more than willing to pay an NSFW fee because I look for artistic skill more than the fees itself.
When I commission pregoo for example, She charges for background and sometimes belly, and I don't mind at all because I prefer the style that justify the amount required

I don't really agree with the sentiment if it is based on difficulty, since fabric would be more complex, and would be a larger contributing factor to the work. If it is an artist that doesn't make lewds on average, or at all for themself, and/or morally objects to it then I can't fault them. However I am probably not going to get the NSFW commissions from them, either just asking for a skimpy bikini or the like, or just hire someone else who is ok with drawing lewds.
Stuff like fetishes I might make an exception with some kinks that are more "dry" of artists then others, but it would depend on the artist style, quality, and the price.
If the costs are high enough I would just make my own art if it came down to it.

Commissioner here but I've been commissioning for a couple of decades now and have talked to a lot of artists over the years.

One thing about NSFW fees is that it encourages more SFW content for the artist; which in turn is work that they are more able to use for portfolios etc. NSFW content is not something you usually want to show off for most conventional work examples.

Sounds like an excellent plan.