I was reading a discussion where several women who are transgendered noted that since beginning HRT their preferences for sexual partners have changed. Most of them considered themselves bisexual before and after starting HRT, but had noticed a differrence.
I have no idea if this is totally off the mark, but some of the responses to the question make me think of research into how a woman's position in her menstrual cycle affects the type of men she finds attractive (Apparently the studies assume heterosexuality).
When a woman is most fertile she prefers "manly" faces and smells much more than when she is relatively infertile. This strongly suggests that sexual preference can, at least to an extent, be modified by hormones.
When MtF people are given hormones, they presumably mimic the hormone loading at one point or another of a cisfemale's cycle. It seems to make sense that this could cause at least a small change in mate preference.
When Fertile, Women Want Manly Men
What do women want? It depends on the time of the month
Women's Preferences for Male Behavioral Displays Change Across the Menstrual Cycle
And for the guys:
The face of fertility: why do men find women who are near ovulation more attractive?