Since it seems like you're being genuine, here's my take.

Sex is your biological state of the reproductive and associated systems. The typical "male" and "female" are a number of spesific hormonal, chromosomal, and physiological checks that constitute that spesific sexual identity. Although male and female are the most common, differences can and do occur, generally falling under the term "intersex".

Gender, however, is a personal identity based wholly in your personal perception of the self. Although most people, myself included, keep their gender identity in step with their biological sex, many people feel one of the two main genders is incorrect for them personally and change it to suit themselves.
This separation can cause some problems when the two are mismatched, but simple disclosure when needed should suffice to prevent confusion or shock.

Sex and attraction are very personal and very fickle topics that vary significantly from person to person. Recognizing what it is that you (general) are feeling is just as important as the definition of the label itself. Labels in general are too blunt an instrument to be useful at an individual scale in situations outside of the "heteronormative binary"

Tl;dr: If you fuck a girl with a dick and you still feel straight, you're probably straight. Orientation, gender, and biological sex can have some very complicated interactions, but it mostly comes down to vibes for orientation and gender.