What does it matter?
It doesn't, depending on what a person thinks of the implications. Because Augustine converted with a simultaneous vow of celibacy, all of his sexual activities are strictly pre-conversion. It would be hard for GLBT groups to use him as much of a poster boy for their causes considering he in no way condones his own sexuality, or hardly anyone's sexuality for that matter. In Augustine's eyes, sex is worldly, temporal and usually lustful, so any attempt at expressing acceptance of any kind of pleasurable sexuality is cut off before it even begins.
For some Christian groups, it might be unsettling to imagine their patron saint engaging in sexual acts with other men/boys and I would think many would resist any idea speculating about Augustine's bi-sexualty.
On the other hand, I can imagine that some Christian groups would gladly take Augustine as the prime example of an "ex-gay/ex-bi" person whose behavior was changed by belief in and commitment to the Christian God.
Augustine could be co-opted by either side of the divide.