Was the extent of the molestation by your cousin watching him masturbate? Was there any physical contact between the two of you? It sounds like you really got aroused to the sight of his masturbation and it was a powerful experience for you.
On that same trade, ordinary plain old nude photos of women, or sometimes even if they're clothed, get me very aroused. Naked pictures of men, no matter how good looking I acknowledge them as being, don't turn me on at all. But if we cross over into the video form of media, it's a fucking crapshoot.
The lack of reaction to still pix of males is not the same as the action shots (like you witnessed with your cousin). Like your assessment as a Kinsey 2 (predominently heterosexual?) and the above comment, I'd say you appear to be heterosexual but had a strong turn-on to watching your cousin. Making that kind of connection is something that a lot of survivors have around here and causes confusion on orientation.
Can you/do you fantasize about females when you masturbate? What's your relationship like with your cousin today? Do you think you could talk with him about this?
This should open some discussion here with other guys having struggles with porn.
Hope you get some answers.