This question is tricky because it's not clear that Halachah recognizes such a phenomenon of one who is just asexual though technically physically capable of sexual intercourse. Similarly, it's not clear whether Halachah recognizes absolute homosexual orientation as one who not only is irrevocably attracted to men but also absolutely uninterested in women, despite the fact that he, too, is physically capable of sexual intercourse with a woman. However, whether homosexuality itself leads to its own requirements of forbidding Yichud with other males is already addressed. See Tur and SA EH 2424 and BachBach, ChMChM, and BShBSh there, as well as Chochmas Adam 126:14;126:14; Aruch HaShulchan EH 24:6;24:6; Shu"T Daas Kohen (Kuk) YD 3YD 3. If one knows himself to have homosexual tendencies then indeed he must not be alone with another male, especially if the other male also has such tendencies. Regarding an asexual or homosexual man being alone with a woman, it would appear that indeed since he is physically capable of sexual intercourse all Halachos of Yichud would apply. All literature on the issue discuss whether or not one who is physically incapable of sexual intercourse may be lenient regarding Yichud- and even then it's not simple...