sfa made a good point. Just to be devil's advocate - it could be that he really is fascinated by the swingers thing, and that he spends time thinking about it, or has talked to friends about it, but hasn't brought it up because he was afraid you'd be upset. He could be dreaming about doing this with you, but just can't get up the guts to talk about it. So he's satisfying his curiosity and is embarrassed at being found out. So how would you react if you went to counselling and this is what he said? (You don't have to answer, I'm being retorical here).
But the lying bit is bad. It doesn't sound like he is trying to be honest with you, or attempting to make things work. You don't have to tell the whole truth all the time, but outright lying is not a good thing and shows disrespect most of all.
What is a dealbreaker for one relationship can be just petty details, or even a fresh start for another one, but large portions of trust, honesty and respect are pretty basic requisites for a longterm healthy relationship. You have to consider if the two of you are willing to work on this I guess.
Winter riding is much less about badassery and much more about bundle-uppery. - malkin
1995 Kona Cinder Cone commuterFrankenbike/Selle Italia SLR Lady Gel Flow
2008 white Nakamura Summit Custom mtb/Terry Falcon X
2000 Schwinn Fastback Comp road bike/Specialized Jett