I think it's both actually. I Believe one can be born homosexual and i also think how you grew up plays a MAJOR role in your sexuality.
For me i think it's because of my family. My family doesn't have a lot of guys in it. I have a load of uncles and female cousins with whom i used to be around all the time as a child.
some straight guys test their loads to see if it's sweet or salty.