Actually, it did seem to be at least hinted at that Brynden Tully was gay. Remember that Hoster Tully called him "the black goat of the Tully herd" for not marrying a lady, implying that Brynden is different from the rest of the Tully family in some way. Homosexuality is a possible explanation for this remark, but as the all the facts on this exchange have yet to be revealed, it is just fan speculation.
What's odd about him not taking a wife, is that the Tully words are Family, Duty, Honor. Being of noble birth, it is expected of Brynden to marry, thereby making it his duty to his family to wed a woman of high birth. Refusing to even consider marriage does raise some eyebrows. Again, this is just speculation.
However, what this boils down to is that the author(s) of this mod, after presumably doing research into the world of ASOFAI, came to the conclusion that Brynden Tully was at least a little queer (as in odd), and that the homosexual trait best reflected Brynden's personality as the game and mod currently stand, at least for the time being. Hopefully this helps in some way.
Being gay doesn't necessarily mean he is any less of the man's man that we all know him to be.