Trans women tend to transition after having had experience with women, and it's not rare that they have an active relationship with one.
I think all that proves is that experience and environment play a non-negligible role in sexual orientation after transition. That, and exposure to T probably increases the chances of feeling attraction towards women. They were interested in women "because of T", then transitioned, and never felt the need to change, since they had "learned to love women" already.
On another field, maybe being into women is an encouragement for some to transition. Accessorily, homosexuals tend to be more accepting of gender differences, but most importantly, I think there might be a feeling of moral conflicts in MTFs who are into men. They, or their friends and family, might think that transitioning would be fleeing, trying to "excuse their gayness".
Or, uhm, maybe we tend to want to stay away from men, since our own body could have traumatised us in a way that everything guys do is undesirable to us.
It could also be some kind of craving for feminity. It's lacked for so long in our lives that we would not only seek it in ourselves, but also in people close to us.
And women are thought - the general opinion is probably exaggerated, but I don't think it's entirely false - to be more accepting of differences in people close to them than men. Be it the (discutable) fact or the rumour/opinion, maybe it's an encouragement to lean towards women.
Finally, maybe "moderately dysphoric" trans-ish MTFs tend to transition less if they're into men, since being gay could put them in an environment where, sexually, they can probably imagine feminity from the gay bottom's point of view, and socially, they can be feminine with less social repression, because of the preceedingly named more accepting environment there is in homosexual communities.
It can probably explain at least part of the gap... Maybe.
