It's up in there air as to whether Cloud and Tifa did it. To be honest, I didnt suspect it until I read this thread. If they did, I assume it was an act of love, intimacy, and comfort, at the very least. They were both hurting from losing Aerith (Tifa cared about her too, even if they werent always on the best of terms considering the love triangle), worried that their friends were never going to come back, and aware that they could die very very soon. I'd definitly wanna consumate my love if it was possible that I was gonna die the next day.