Marriage Realities

I've been married to Helen for twenty years now. We have a good life together, two kids in college and a cozy house by the lake where we spend our summers fishing and watching sunsets. Life is pretty idyllic on paper. But recently, I've noticed changes that aren't so rosy. Helen used to be more adventurous in bed, she would whisper naughty things in my ear while making love that sent shivers down my spine. Now it's hard to even get her naked before we fall asleep from exhaustion after a long day at work. Our sex life has dwindled to just the basics and I've started wondering if there might be something more out there for me. So a couple weeks ago, I found myself in an online chat room with someone twice my age. She was playful and flirtatious, which is something Helen hasn't been in years. We ended up exchanging phone numbers and talking every night before bed about our fantasies, she'd tell me stories of her younger lovers and ask me to describe my sexual past. After weeks of chatting I finally asked if we could meet up somewhere private next time she was in town. She said yes, and we went straight for a hotel room where she immediately seduced me with whispered dirty talk about how much she wanted to be my wild lover again. We had amazing sex that left both of us shaking, she told me it was the best night ever. But after I came home late last night from our encounter, Helen seemed distant and asked if everything was okay. Now we're arguing about whether having an affair is justified because our marriage has grown stale or if I should be more content with what's right in front of me. Am I being unfair? Do other people have similar struggles when they feel like the spark between them and their spouse has died out?