Hum.... I think we don't have enough info to know.
We know about Kilindi (she is not canon, though, but she totally should be, lol), and we also have Eldra Kaitis, who might be a love interest.
That's what we know from his youth.
So we have info about him being at least straight. I do HC the nightbrothers and sisters were deep into bisexuality because of how they lived and how their culture was, and had Maul been raised with them he would've probably been at least bi, but it isn't the case.
Then he was cut in half, which means he has no genitals so he probably he doesn't feel or want anything physical... I wonder how this affected his mind though, not only for the trauma but also because he probably wanted sex at some point but he couldn't have it.
We also have characters that may feel something for him, perhaps Rook Kast or Qi'ra (though I seriously doubt about her), but we don't know his feelings towards them, so it's difficult to tell because there's not enough info. (And it's kind of frustrating!!)
So I HC after he was cut in half he might be ace?
I don't know, what do you guys think?
Thanks for the ask, btw! 😊