Greetings:
First of all sorry for my lousy english. Last night I uploaded I video called "Straight guy evolution 14". The clip shows a guy increasingly 'committed' to gay sex. An user/moderator has sent a message saying the category was wrong and telling me that I should have included it into 'Straight' or 'Bisexual' category. However only 1 hour of content is straight oriented (the video has a duration of 4 hours). Question is : which category should I include it?

From what I can see.. that video should be in the ASIAN category.  As a sub category, you might put it in bisexual.  It should not be in STRAIGHT. 
You uploaded straight guy evolution #14
I also took a look at #13 which seems to be  shemale / trans
#12 is asian. 
#11 is BDSM / bondage
#10 is asian  - bi

Almost all asian videos - including those, have digitized genetalia, so the most important category to put them in is Asian. 
I always found it strange that Asian videos are loaded with degradation, BDSM,  scat, and what looks like some underage performers.. but they are adamant about digitizing genitals.. even on cartoons!