Here, according to Marince Col. Steven Day, the judge in the case, the gay porn found on the Navy doctor's computer was admissible to establish motive -- a "possible need on his part" to view "young, athletic males" engaged in sex. Of course, this is not evidence of "motive," at all but instead (fallacious) evidence that because the Navy doctor apparently liked to view gay porn in the confines of his home, he was more likely to have broken the law and breached the trust that he had with his colleagues in the Navy by recording them while engaged in the most intimate of acts. One wonders whether the same ruling would have been made in a case where a Navy doctor had heterosexual porn on his computer and was alleged to have recorded midshipwomen having sex.
Courts have only admitted evidence of past sexual acts for other purposes such as motive or common plan or scheme when "it related directly to the victim or was the same type of activity that the defendant allegedly performed with the victim." United States v. Gillespie, 852 F.2d 475, 479 n.2 (1988). Even then, courts have been particularly wary of allowing such evidence because "it is extremely prejudicial." See id. at 479.
Of course, in this case the porn on the Navy doctor's computer was neither related to the midshipmen he allegedly videotaped and it was not the same type of activity as secretly videotaping sexual acts. Furthermore, the prejudicial effect in a military case of evidence of homosexuality is almost certainly higher than it is in the usual case. All this adds up to a horrible ruling on the part of the judge.
As EvidenceProf remarks, one really has to wonder that if the porn was heterosexual, would the same judge think it would demonstrate a predilection toward secretly videotaping women disrobing or it would just be a case of "boys being boys?"