It was a prank.

Leo had been dared to ask you out. To spend all that time with you. To kiss you under the stars, and offer you an umbrella when you got soaked by the rain.

The Aphrodite campers had been gossiping about it. They told you everything... you had been dating Leo for way too long, and there was no doubt you were in love with him... but he must not feel the same way. It was a prank, to see if he could date you.

...but it backfired on Leo, horribly-

Because he fell head over heels.