The best way to leave a lasting impression is to be as romantic as possible. It's good that you came prepared. A little studying can go a long way.
“Pinkie, I just want you to know how special this night has been. I wrote a rhyme all by myself, and it's just for you.”
Pinkie leans down and rests her head on her forelegs. She looks up at you with a smile on her face.
You clear your throat and begin.
“The sky above can't match the depth of the beauty in your eyes.
Your voice is calm and pure, singing like a hundred lullabies.
A hundred thousand bits couldn't buy such lovely skin.
Your hair is smooth and clean, clear as a beloved violin.
“When the sky's blue, and the wind's calm,
I see your face and I have no qualm.
When the wallet's full, and the music's right,
I see your face and it is bright.
“Your happiness is my happiness as our futures are intertwined.
Well, what I'm trying to say here is this: will you be mine?”
“Aw, that's so sweet.” Pinkie blushes. She holds out a hoof and opens her mouth, but another pony speaks first.
“And it was written by somepony else.” Twilight walks up, giving you a dirty look.
“Uh, no,” you say, “I know it sounds a bit like Cantering's Woo, but I only used that for inspiration. If you compare them, you'll find that they're completely different.”
“No they're not! You recited his rough draft! That's exactly what he wrote word-for-word.”
“Wait, you mean you didn't write that?” Pinkie puts down her hoof and frowns.
“Uh, sorry, Pinkie. I just thought it would be romantic and wanted to do something special.”
Twilight snorts. “Well, the poem is romantic, but your claiming it as your own is a huge no-no. If you were honest about who wrote it, then I wouldn't have a problem. But now? Pinkie deserves better than a plagiarist.”
“Wait, you're going to give up on me over that?” You ask.
Pinkie sighs. “Well, yeah. We had a lot of problems with Suri Polomare who copied Rarity's dresses. I don't really want to associate with somepony like that.”
You're crushed as Pinkie walks off and Twilight follows. Why did you have to be so insecure as to copy somepony else's work? Why?