The answer to this question is complicated and depends on a number of factors. For some men, engaging in sexual activity with a transsexual woman may simply be a matter of physical attraction - they find her appearance to be attractive and are sexually aroused by her. For others, there may be a more psychological or emotional component to their attraction. Some men may be attracted to the idea of being with a woman who has a penis, or they may find the transgender experience to be exciting and novel.
Regardless of the reasons behind a man's attraction to a transsexual woman, it is important to remember that everyone experiences their sexuality in a different way. There is no single answer to whether or not a man who has sex with a transsexual woman is straight, as sexuality is fluid and personal.