I don't know what she's reading, but I've read a LOT of slash - I mean a LOT - and I don't think I've ever seen anyone "turn straight," although I suppose maybe it happens in some fic I've never seen? (Maybe it's mixed in with those I choose not to read for myriad other quality-issue reasons.) Maybe she is referring to OT3s and multiple relationships at a time - you know, like the J/W and W/E series I'm working on. Maybe she's misinterpreting and doesn't understand, for example, that Will would be carrying on with both of them at once - just not together in the same room.