I think that line is the most telling from your post. This isn't just about his interest in swingers--that in itself could be anything from what he says it is, just curiosity, to something that endangers not just your marriage but also your health. And I could even understand him wanting to hide it from you--it's not a curiosity most people would be proud of. But it sounds like there's a history in your marriage of lying, and that has eroded the trust that is absolutely necessary in a marriage, so you're left in this situation where what he's doing could possibly really be no big deal (curiosity) and could possibly be a really really big deal and you have absolutely no way of knowing which one it is because you can't trust him to tell the truth about it. And THAT is a really really big deal.
It's up to you to decide if it's worth fixing. Counseling for yourself would be the first step, and marriage counseling as well. It will probably be a long and difficult road, and no guarantee that things will get better in the end. Honestly, if I were in your situation with no kids and a history of problems, I'd be inclined to cut my losses and leave, but I'd want to try counseling first to see what comes up--it's entirely possible that I'd find out that *I'm* the problem--someone with control and trust issues (not saying that's you--I'm saying that would be ME in all likelihood) and I'd want to know that either to save my marriage or to head into a new relationship with a better understanding of myself.
You also asked what we would think if we found out our husband had filled out a profile on a swingers website. For my relationship with my DH, I'd think "ewww" but it wouldn't go any further than that because I do trust him, and he has been honest with me throughout our relationship, both in little and big things, so if he told me it was curiosity, I'd believe him. I'm not saying he's perfect, and I know he lies about some things (everyone does) but not about anything important--and by important, in a strong relationship that means important to BOTH people. Even if your DH thinks something isn't a big deal, if it is to you, then it should be to him too.
Sarah

