Just provide them with evidence that you've been in each other's company within the last two years (i.e. flight itineraries, passport stamps, photographs of you together for every trip you've made to see each other) and that you have a genuine relationship (letters, cards, e-mails, phone bills showing you called each other regularly) and all should be well. The "intent to marry" part, I think, just means that you have a real relationship that would logically lead to the desire to wed. Also, pay attention to how you answer question no. 18 of the I-129F; your response could (should?) express that you're really in love, have been in each other's company, and how/when you decided to get married.
Good luck! Buona fortuna!
Karen_L got a reaction from kenk0001 in Proof of Intent to Marry