I haven't transitioned, so I wouldn't know personally, but here's an excerpt from the
fmguide.org that I read before
Myth #6: Taking testosterone will make you gay.
Some trans men may find that their sexual feelings and attractions shift after starting testosterone therapy, while others may not. Some trans guys are attracted to women pre-testosterone and remain attracted to women post-testosterone. Some are attracted to men pre-testosterone and remain attracted to men post-testosterone. Some may find their attractions shift from women to men or vice versa pre- and/or post-testosterone. Human sexuality is a complex subject, and FTM transsexuals are no different in this regard. There are straight, gay, bisexual, and asexual trans men, as well as people who don't really identify with any of those categories.
So, there is no specific correlation between taking testosterone and suddenly becoming gay. However, this myth may have gained some steam because a portion of trans men do experience a change in their sexual feelings post-transition. However, this may have more to do with the emotional changes that come with living as a man and being seen by the rest of the world as a man, rather than the simple presence of testosterone, per se. That is, once a trans guy has begun to feel comfortable in his body and in his role as a man, he may feel more comfortable exploring sexual feelings for men than he did pre-transition. Many trans men are uncomfortable in their bodies pre-transition, and as they become more comfortable post-transition, some might feel more confident in new or different sexual territory.
This myth probably also gains some momentum because it touches on some of the fears of the partners of trans men as they embark on transition. That is, many partners may wonder, "will he still be attracted to me?," or "will I still be attracted to him?" Concerns about community identity and loyalty can also compound this issue, particularly for those female-to-male trans people who may have roots in LBG (lesbian-bi-gay) communities.
The truth is, no one can predict all of the things that will happen to an individual during or after transition, and sometimes there will be surprises. However, if testosterone made all trans men gay, then there wouldn't be any post-transition trans men with female partners, and that is simply not the case.