Are you happy in your monogamous relationship sexually? Other than the normal fantasies, what online activity do you engage in? Is your partner aware of it? Have you ever caught your partner online? Does it bother you? I have no problem with a partner looking at porn videos but online dating sites and chat I find a step over the line, unless agreed upon.
My husband and I have been in a monogamous marriage for nearly 30 years. I'm very happy sexually. We went at it like horny rabbits from day one, and honestly it's never slowed down since. If anything, it's gotten deeper and even more passionate and intense. Sure, we experience peaks and valleys, but generally we make love at least 3-4 times a week.
As for online activity, I generally like looking at pics and photos of hot men, and I know he does the same. Doesn't bother me in the slightest. We both love looking at men/male anatomy and will often point out hot guys we see in public. He knows I'm an ass man, so he knows the kind of guys who will really catch my eye.
Aside from forums such as this one, I also chat privately with several guys, but they all know I'm in a sexually monogamous marriage. I don't think there's anything wrong chatting with someone and even engaging in explicit sexual talk as long as it doesn't cross a boundary, and for me that would be expecting to hook up, which I'm never going to do.