His fantasy is to be a lady. Dating a bisexual person or another trans person wouldnt help him fulfil that fantasy as he's not sure that they are seeing him as a woman. Same reason he wont date people who are specifically interested in transexuals. Only dating an ostensibly "straight man" can fulfil the fantasy. Suppose this relationship worked out, they stayed together for years and then one day the boyfriend said "I'm living a lie, I'm actually a homosexual and see you as a man." Literally nothing would have changed but the relationship would be over because the trans person is no longer having their fantasy of being seen as a woman fulfilled.