Here's the text from the "Holidays" card:
As we each celebrate with our families, I will be thinking of you. I will miss spending the holiday with my special lover...the one I really want to be sharing it with...These are the times when I wish we could be together and not have to hide our love. Until we can celebrate together, my holiday will be incomplete without you.
Let's leave aside the sleaziness of the concept for the moment, and just think in practical terms. You're having an affair, which suggests that you don't want your (legal) significant other to know about. So why on earth would you send your paramour greeting cards that s/he can use to threaten you with the next time you have an argument? "You're buying me that diamond brooch, or I'm showing these to your wife!"
And getting back to the sleaze factor, these cards are a celebration of immorality and cruelty. Not everyone is into monogamy these days, and I don't object to whatever limits (or lack thereof) a couple chooses. But if a couple has agreed to a monogamous relationship -- and why would you need to keep your secondary relationship a secret if you hadn't? -- then adultery is a violation of that relationship, and it should be condemned, not celebrated "with empathy and understanding." The "Secret Lover" collection is repulsive.