...Nah. Laguna's encounter with Julia was long before he met Raine. It's doubtful even that he and Julia spent the night together.

Laguna and co then travelled all the way to Centra, to the excavation site. Quite a long journey. Laguna was injured, and eventually made it to Winhill. He spent six months recovering from his injuries. When he never returned, Julia was lonely and eventually married Caraway. If Julia was pregnant to Laguna, she'd have been more than half-way there by the time she even met Caraway. Very little possibility of mistake. Laguna was bed-ridden with terrible injuries for a long while; unless Raine raped him in his sleep, it's doubtful that Squall was conceived until quite a long time later - not long before Laguna left to find Ellone, I'd imagine.

I've heard this theory before, but the time and circumstance differences make it exceedingly unlikely, even impossible.