I think you misinterpreted the post. Dobson didn't say that Alex became a lesbian, he said that a strange relationship formed between her and Atea. They both knew that Alex was straight, and it was only through this back-and-forth that they realized Atea was gay for her.
The way I'm reading this, the two were basically doing a form of roleplaying over AIM-- I don't think it was full-on dialogue exchanges and prose, but they were spinning out little stories together with Sennwald 'playing' Alex and using Dobson as a sounding board, to which Dobson would in turn 'play' the other characters, starting with Atea. Who had a 'strange relationship' with Alex that they both realized was romantic.
In other words, Dobson wound up expressing his own feelings for Alex Sennwald through use of this character and played it off as Atea just being a flirtatious bisexual woman. Hah hah, yeah, it's... it's the character. Yeah.
I doubt that Alex fielded the full-on 'sex criminal' form of Atea, or that Dobson would have been forward about using her that way, but the more I think about it the more uncharitable and outright creepy the situation becomes. Alex got engaged and Dobson drifted away from her, and in the process retooled the characters to a pirate adventure and along the way turned his more personal character, Atea, into a sex-crazed maniac that Alex (despite not returning her feelings) would always and forever put up with even though Alex was deeply uncomfortable with it.
There's a lot to unpack there.