Sex is what you are born as. Gender is what you identify as (i.e. born a man, identify as a woman, or non-binary, etc.) Gender identity is more based around personality and what attributes define you.
Based on the best clinical research we have, sexuality is not binary. Meaning that we aren't either Gay or Straight, we can be varying shades on a scale: See
here
There is a lot of variation out there around what homosexual couples are attracted to. My sister is more of the traditional woman role and her wife is more like a man. However, it's not always like that either. Society has created these social constructs on what defines a man and what defines a woman. We have even created gender roles based on those assumptions.
Personally, I am a man in sex and gender, but I find friendships with women much easier as I identify with their mindset more. Does that make me less of a man or less "manly". I don't think it does, but based on gender roles and the traditions and mores we have instituted as a society, it might appear that way.