I care for my wife of 30 years, but I don't feel passion for her anymore. It's like I'm making love to my sister. Recently, I met a female penpal that I have been communicating with. A simple touch from her has created sparks for me. We humans are not totally monogamous, but slightly polygamous.
All marriages have their hills and valleys of contentment in and out of the bedroom.
In long-term marriages, the equation can often be that familiarity plus routine equals boredom. Boundaries need to be created. To keep your feelings secret will only intensify them. Take responsibility for your feelings and work towards fulfilling your needs. You could also try a relationship review where you go back to the early years and remind each other what turned you on.