Yeah, I agree with caution on the rumours. It's more like elements of what's coming out are true but people are blowing it out of all proportion, misinterpreting things, and running away with it
Apparently from what's coming out now, there are apparently two different plot threads a) Tidus/Yuna on the island and b) The past - female summoner/male guardian/male summoner, male summoner is now an unsent and Yuna meets him later on the island or something (or she meets him earlier on the island, he explains everything to her, and how they used to do it in the old days, and what she might have to do if something happens to Tidus again, which might go towards helping to explain her attitude to Tidus while they are on the island, etc )
I am kind of confused but I think what it means is that female summoners might have needed to have sex with male summoners/vice versa to create fayth (Yunalesca rule)/ancient time rule, not that Yuna had to have sex with Tidus's ded headless body to bring him back or whatever else others are posting/running away with elsewhere, you don't have sex with the fayth/the person becoming the fayth/is a faith but with another summoner, of course this begs the question did Yuna have to do that now this time, or did she find another way.