As I told elfling_eryn it would have been creepy and exploitative for Sephiroth to have sex with Cloud any earlier in the story. I originally planned to skip the bits that show why Cloud trusts Sephiroth, but they wanted me to go into detail about introducing Cloud to the harem and I knew it was going to be too long for a single post, so I went with it. And my original concept had Sephiroth actually fucking Cloud their first time, but he told me in no uncertain terms that he wanted to do research first - he didn't even realise that he'd need to prepare Cloud beforehand - and Cloud's interest surprised him.