The difference is one of love,wanting and trust. You've got to want it or it doesn't make sense to marry. Don't marry if you don't want to. Don't marry if you still want to screw around too. There are open marriages but they don't usually last. You've been with this person for 3 years. You don't have to marry but you've got to decide if you want to stay with this person. If you're looking to relive the screwing around part of life and your love for this person doesn't get in the way you need to be honest and leave. Above all be honest with yourself.
Desire does wain over time with age. But you're 25' stop masturbating if your desire to have sex with this person wains. It helps to think of the goal of sex is not to come but to make your partner come. Part of any committed relationship is wanting to please your partner. Pleasing them can be very fulfilling.