After several years together, we're often disappointed when we admit that our sex lives aren't what they used to be! Face facts: sex has got boring. You want sex, but unfortunately your partner rarely crops up in your sexual fantasies!
There are many reasons why you might be tempted by another sexual partner without necessarily wanting to leave the one you're with: ingrained habits, fear of telling the truth about how you feel, tenderness taking the spice out of your sex life, a thirst for something new, the inability to play out your sexual fantasies, etc etc.
It's vital to separate love from sex in such cases, otherwise you'll be all over the place and you'll never know what your true feelings are.