AFF is very much oriented to swingers and open relationship types, along with a few other things. The environment there is very sex-based. There are also a lot of limitations on the site designed to make and keep people paying. So... yeah. No one reads profiles, since generally they can't see them. The polys are probably there, but I think many lose interest after a while of dealing with the same issues you mention.
There are other free sites like PlentyofFish and OKCupid. POF is fairly mainstream, but there's probably a slightly higher poly ratio there. The FSF section also has a fair number of couples seeking unicorns, so you may find some similar situations as AFF, but at least you don't need to pay for it.
Generally, OKC seems better recommended, as the matching algorithms, while fallible, still tend to trickle like-minded poly folk to the top of your matches.
Yay! It sounds like I've got lots of online dating ideas. Now to move on to real life.
I wanna know how some face-to-face encounters have gone. What's worked for you guys? How do you approach someone and then later tell them to meet your girlfriend?
Story time!

Ben works at a bar. A girl dropped him her number! He told me she was really cute, someone we'd both be really interested in, and so he started flirting with her via text message and still thought she was really cool. Finally she asks Ben, "Are you seeing anybody?" and Ben said something along the lines of,
"Yes, but we have a very unique relationship."
"Oh yeah?"
"Yes, she knows about you and she knows we've been texting."
"What's so unique then?"
"We're both looking for a girlfriend."
We didn't get a response back from her for a while. It was so nerve-wracking. She finally explained that she had been super curious about sleeping with girls and that she was currently with a wide-open lesbian who actually made her nervous.
She said she was really interested in Ben and me, because it would be an easy way for her to dabble into girls and still have the comfort of the opposite sex there.
Finally, we got the balls to ask her out for dinner, something casual, so I could meet her and whatnot. She said that this week wouldn't work and that she'd call us when she might be more available. Haven't heard much from her since except she "accidently" sent Ben a text. Now I'm just worried she's just in for my boyfriend. But she told him that she still interested in dinner for three.