I wasn't stating it as a fact, just a suggestion.

I've been thinking back through the timeline, and Aerith's adoption would have been about 15 years ago. I don't know how Jessie and Biggs could look the same 15 years on. They couldn't be that old. Barret was only 20 at the time, and his AVALANCHE hadn't formed yet. I believe he found Biggs, Jessie and Wedge studying Planet Life at Cosmo Canyon 4 years prior to the game.

It may well be that Jessie and Biggs were in a relationship 15 years before, but I don't want to say for certain.