If a straight woman wants to have sex with a trans-woman, it means she actually sees him as a man, which isn't how she wants to be seen. The relationship would be based on, from the trans-woman's perspective, her being misgendered. At best the straight woman is into the trans-woman because she doesn't understand the trans-woman's identity... or worse perhaps she has a fetishized view of them.
Perhaps she wants to have sex with him/her/whatever based on first impressions without knowing them. That does happen you know, regardless of gender, gender identity, sexual preference, etc. Peoples opinions change after they get to know someone, in all sorts of ways.