What is my sexuality if my boyfriend is transgender (ftm) and I am sexually attracted to him but I am not attracted to females?

Your sexuality is determined by the gender of the people you are attracted to, not the biological sex they were born with. If your boyfriend identifies as male, and generally you are attracted to men and not women, then you're straight if you're a girl, gay if you're a boy.

Your boyfriend is a man -- trans people are the gender they identify as, and science continues to confirm this when comparing the brains of transgendered and cisgendered men and women. If you are a woman who is attracted to men, that generally would make you a straight/heterosexual woman. If you are a man who is attracted to men, that generally would make you a gay/homosexual man. That said, there's really no need to worry about labels unless you find having a particular one to be comforting or affirming to your experience. You should date people you are attracted to and who you have romantic chemistry with -- regardless of their gender or the sex they were assigned at birth.

I think all that's important is your relationship. Try to just focus on the happiness and love you share. I wouldn't get caught up in sexuality because it is really just a label when in reality sexuality is not black and white or clear cut but more of a spectrum. Not everyone will understand you but that's not their business. Sex and gender are not the same thing so if someone identifies as male then you are therefore attracted to a male so it doesn't make you lesbian. Just because he was born with the wrong parts doesn't mean that he is less a man than a man born with all the male parts. Some people don't bother identifying as anything because sometimes it's just best not to feel bound by gender.

You’re whatever sexuality you see fit to identify with! Sexuality is a spectrum that slides wherever you see fit. If you identify as straight, perhaps you find your boyfriend attractive because he identifies as male whereas other cis-gender females do not. If your brain sees him as male, it will not account for the fact that he was once female. In any way, if you have found someone that makes you happy, worry less about the sexuality you fall under and more about how happy the person makes you. Talk to him about it! Talking with your partner does wonders!

Your sexuality doesn't need a label! It is up to you to determine how (or if) you want to define your sexuality and no one else can do that for you. Being attracted to the opposite gender but not the same gender is usually defined as heterosexual or straight, but if you don't feel that way, then simply don't use that label. You can also be attracted sexually and romantically in different ways. For instance, if you are attracted to all genders sexually and only a few genders romantically, you could define your attraction differently to both. I am both pansexual and panromantic, so I am attracted sexually and romantically to all genders regardless of their gender (but not all people!).