There are a number of theories, all of them at best semi-speculative, some semi-NSFW as well. (I've lost my source for some of these, I'm sorry).

-It is the image of a heart, sliced in half. It's not a close match, visually. Plus, this image originates in an era and place in which dissection is verboten.

-It's buttocks, as seen from above. Odd perspective, and why would it be pointed?

-It's a pair of breasts. Again, weird perspective and an unexplained point.

-Gloria Steinem argues that it's a "residual female genital symbol," perhaps the spread vulva.

-It could represent the seeds of the Silphium plant, which was a Classical contraceptive related to fennel, now extinct. It's true that a similar shape appears on the coinage of the city that traded in it, and the plant was symbolically related to sex and sexuality, but the link between the shape and the emotion wasn't made before the 14th century, more than a millennium after the plant went extinct.

Are you specifically referring to the usage of "<3" as representing the heart in textology, or as in the usage of love?