There is no doubt form the start of the movie that Forrest loved Jenny from the beginning of their friendship. But it is Jenny who never gave a thought or had a feeling about Forrest's feelings. She always avoided him right from the start and considering girl's sixth sense is too strong, she most probably understood that Forrest started loving her. But watching the movie, somehow it was depicted that Jenny despite understanding everything (even Forrest expressed himself that he loved her) always tried to ignore him (rude to say but the reason being he was stupid and this is the truth) and chose an aimless yet apparently enjoyable life for her.
But in the end she returned to him conceiving how much life she had spoiled of herself. She was sure he would accept her in any condition, had a baby with him for her own interest (for natural reason) and married Forrest when she became sick. Needless to say Forrest had become too rich by then. Is the above theory not enough to prove that she married Forrest for her own interest?