Agreeing with the rest here. Sexuality has got nothing to do with gender. Cis males and females can be straight, gay or bi without questioning that. Being trans is a gender issue and has nothing to do with sexuality. It CAN affect perceptions though or there can be confusion.
I can say that personally I'm now coming to terms with the fact that I actually didn't find guys sexually attractive when I thought I did; I wanted to BE them. I wanted their hair cuts or the clothes they wore. We don't fall for someone and then try to dress like them, right? That's kinda crazy, but for a long time I was just confused and mistook attraction with jealousy. So sometimes the process of transitioning or accepting being transgender can actually affect sexual interest. However, at the end of the day, being trans isn't any different to being cis. You fancy who YOU fancy. A criteria of being trans isn't that you have a particular sexual orientation because to assume so would assume that all females and males are just hetero.
IMO there's nothing to explain. I find that when I come out to some people they very often ask stupid and irrelevant questions, like "Do you fancy girls then?" or questions about downstairs. I find when people ask things like that it's just worth politely pointing out that neither of these define gender. A man can be straight, gay or bi. A guy can also be without genitalia through trauma. Do any of those make them less of a man? If anyone every responds "Yes" to that question, well I don't want to know them anyway.