Surely if married men (by which I mean the ones who admit it on their profile) were after "a bit on the side", it would be easier for them to join a regular dating site and pose as a single.
For myself, and I guess a lot of the fellas, single or married, the appeal of swinging is threesomes and moresomes. Afterall if you are married the likelyhood is that you have a fair amount of sex at home anbyway. Therefore swinging is an extra bit of excitement and social fun. Doesn't that make married men more genuine rather than less?