The Doctor doesn't have much fashion sense for one thing. In the show he generally picks up an attractive young female companion, shows her around the galaxy, then no sooner after he has dropped her off on a convenient planet, he picks up a replacement. Sounds like a straight guy to me.
With Captain Jack the attraction is quite a bit more obvious, and of course Torchwood will play up to that demographic. It is reported that RTD discusses why WHO has so many gay fans and talks about Captain Jack in this week's edition of The Pink Paper #887 which is a free paper available all over the UK. Sorry can't get my hands on one here in the US and the two-page article is not at their online site, so UK readers feel free to give us the inside scoop in the comments below if you run across one.
I suspect most gays here in the States, like everybody else, could care less about Doctor Who. But that could all change with Torchwood. Captain Jack might be the next Xena!
Late Update: RTD will talk Torchwood in the forthcoming issue of SFX magazine according to this report.