I think you should pay closer attention to him, he lived as other Mortals did, implying he's been various other people over the years, some could have been straight, some could have been gay, some could have even been women.
As to the current body he uses, the Garlean 'Solus zos Galvus' body that he seems so fond of he recreated it on the first. His sexuality is never confirmed, the actors who voiced him played him gay, the body mannerisms and little quirks of behavior were very much played gay, but he did father at least one son as Solus zos Galvus. However, this doesn't imply he was straight either; gay men since the dawn of time have fathered children, and Bisexuality is a legit orientation.
The character was played gay, or as we in the LGBT call it "Coded Queer", but he never explicitly states his orientation, in fact, there are a lot of coded queer characters in this game. None of which outright state their orientation. However, there are out hetero couples throughout the game. If they would just have one or two of the coded queer characters come out it would be a nice touch.