If they'd kick you to the curb for having feelings about them, well, would that really be a situation you'd want to stay in?
I was actually in kind of a similar position a couple of years back. I'd been hooking up with my two friends, Gia and Eric (see subject line). After the first few months, Gia brought up the fact that she thought of her and me as girlfriends (talk about being direct!) but clarified that Eric was not looking for an emotional relationship. This was cool with me at first but I began to develop feelings for him. It started to really bug me that he and I had never talked directly about it but I didn't know how to bring it up.
Finally, I who wrote him a very long email about communication in our friendship and how I wanted to make sure that he and I could speak freely about anything, like for example sex stuff -- I'd kind of like to get it rougher now and then! -- or feelings -- I didn't know firsthand where we stood on the subject of feelings! So, I sort of hid "I'm hella into you" in there as an implication along with other things. He seemed to get the message because he wrote back a very clear email where he addressed all my points -- sure, rougher sex is cool! -- and explained in no uncertain terms his lack of desire for a romantic relationship -- on the subject of feelings, we stand firmly in the land of friendship!
So that was that. Normally in cases of unrequited feelings I'd just remove myself from the situation -- best way to get over it is to distance yourself. But that would have severely impacted my relationship with his wife -- she has anxiety issues and, at the time, felt much more comfortable opening up sexually with him there, plus they're just together a lot generally. So, I stuck it out, kept reminding myself over and over that he'd said "no", started dating someone else (Davis, see subject line) in addition to Gia... not as a way to get over Eric, but it did help... and now I actually can just see him as a close, sexy friend and not get emo about it.
Um, I'm not sure if that story was helpful but it just started coming out? Sorry if it's not super relevant. Anyway, I would be direct with them (sit them down for a conversation) if you can bring yourself to be brave enough to do so, or semi-direct (email them and imply there might be a feelings conversation to be had) if you can't. And if it's just going to cause you heartache to be around them if they really do just want sex, I would step away, start seeing them less. There are other individuals and other couples who WILL be willing to do something deeper with you.
They might need some time to talk this over amongst themselves, so if you do choose to go the direct route you could still consider bringing it up in a light way to start and not having the full conversation right then. If they're not totally oblivious and you're fairly straightforward chances are good they will pick up on what you're saying and discuss it in private, and then potentially be more prepared and less freaked out when you guys have the "real" talk. If that makes sense. In complicated situations like these often people don't know what to say because they doing know exactly how they feel, they need time to process.
It might also help to be clear on what you're asking. What would dating mean to you? Social outings that don't involve sex? Hand holding? Confessions of tender feelings? Or just the openness to knowing that these things could potentially happen? That's what I'd be going for, is the openness to know that it's ok if you have feelings and even show them, and that there's some level of interest in reciprocating on their part, rather then anything specific. Social outings, aka dates, are fun too.
Good general thoughts for your situation here: www.morethantwo.com/coupledating.html