Hmmm, I'm surprised to hear this. (I, too am a straight gf of an FtM.)
I think we are very numerous, but maybe not as obvious or visible. My bf is semi-stealth, long-ago transitioned, and no longer much cares to go to trans/LGBT events, for his own reasons. To the world we look exactly like every other heterosexual couple. There are thousands of couples just like us. We are maybe not as noticeable simply because we blend in to straight society, and we might be past the stage where we show up to trans support groups, social events, etc. (Personally I would like to be more "out" as a trans ally, because I am kind of a political person, drawn to activism. But I hold back on that where necessary, because my partner does not want me to out him left and right.)
FtMs who are much earlier along in their transition process may be more likely to have a lesbian (or maybe bi/pansexual) girlfriend, who was with him before transition and has decided to stick with him/his transition because she is still in love, even if being with a man is not her (primary) identification/orientation. They (lesbian gf's) are more likely to be ushering their man through transition, whereas we straight women might not show up till after the transition has happened. Therefore, in my experience anyway, the in-person or online support circles catering to mostly still-transitioning FtMs and/or their SOs, may tend to have more lesbian SOs than straight SOs.
I wonder though, is there something else behind her question? Is she questioning her sexuality, afraid of how she might be perceived, wondering if she is suddenly supposed to identify with the LGB community more, asking herself if she/you want to be out to her friends or family, experiencing some internal homo- or transphobia she wishes she did not have...Lots of possibilities here. A lot of us go through these issues.
Warmly,
Elizabeth