Neither one was dominant....it was a true yin and yang relationship...and that is why BBM exists! 
rikcub I believe you are exactly correct. I noticed this in the movie.
In the beginning Jack thought he was going to be in control. He found out very quickly he wasn't. Ennis was the dominant one even though this was something he had never done before. (Speaking of which, if Ennis had never done this before, how did he know to use spit? But that is another topic.)
But, throughout the movie we see Ennis taking the active and passive roles. The hug was active. Letting Jack caress him and kiss him and "teach" him what to do was passive. In the motel Ennis was passive being held by Jack. In the tent (second night?) Ennis is on top of Jack but then rolls over and Jack takes the aggressive role by being on top of Ennis. Ennis being passive by laying his head on Jacks Chest. Ennis' strong manly personality was aggressive where Jacks was more passive. Ennis being the aggressor in sex. There was none of this BS, your the wife and I'm the husband. It was just two guys who fell in love.
While there is nothing in the story, screenplay or movie, I often wonder if sex was both ways. We do, after all, understand that Jack had to get satisfaction also. (But then again, that is another topic.)
All these things just jumped right out at me. So, I agree rikcub, neither one was dominant. In my mind, that would have made for a permenant and happy relationship had society allowed them to live together.