You are a:

(1) Friend with Benefits

If either party does not consider the relationship to be sexually monogamous or exclusive.

(2) Girlfriend/Boyfriend

If the relationship is sexually monogamous or exclusive. Mutual intent to marry is irrelevant.

(3) Prostitute
If one party loses the right to say no to sex in exchange for material benefit. (This is title agnostic; i.e. it applies regardless of your relationship status.

(4) Fiance/Fiancee
If both parties have mutual intent, agreement and a plan to marry.

(5) If both parties are married to each other.