When there is a love, natually sex involves. When there are lots of sex, natually love increates. When love and sex is not downhill, no need to promise. Because you know, she/he knows, you are loving each other.
When someone asks you a promise for your forever love, you lie if you say you do. I said you were lying not because you weren't loving her/him, but because nobody knows what gonna happen in the future. You cannot love because you promise, but because YOU LOVE!
Therefore, LOVE! Have lots of SEX! BUT try not to promise or ask for a promise. Well, of cause an agreement or commitment is needed for two who live together. However, at this stage, what you get is the priority and the foundation that could generate love, you can never ever promise love is there forever!