Some people are more just scared to leave a community if they were with someone prior to them coming out and stayed with them. They may well treat their partner as a man and see him as a man but being a lesbian may have been a big part of their past and they're maybe not so able to just drop out of being a member of the LGBT community and become just an ally.
To them they may justify it as having fallen in love with a female and then they later became a man and they loved them enough to stay with them. But perhaps they don't have a sexual attraction to other males elsewhere. For example they may not have any man crushes for actors, but fancy just women. This doesn't mean they don't love their partner, but to compare, most crushes are for the stereotypical handsome or sexy individuals. But in reality, we don't all get into relationships with people anywhere near the league of unatainable people we crush for. If you're not a douche at least, you'll fall in love with all sorts of people because in the end personality should win out over what you look like.
And that's why some will still identify as lesbians. They are sexually attracted to women, but not men, except in their current relationship because they've fallen in love with the personality.
I guess to compare, lets use Steve Buscemi. No offense to the guy, he seems like a nice guy, but he's not exactly A list Hollywood handsome. Straight girls are more likely to lust after the Ryan Reynolds or Ryan Goslings (what's with the name Ryan?!) but in reality they can fall in love with the not so handsome. But even if they wound up with someone who wasn't the typical definition of beauty, they probably still won't lust after the Steve Buscemis of the world despite that. They still lust after the hot guys and they'll be their main go tos if that relationship should fail. The person identifying as lesbian may feel that this is an exception, but if the relationship was to fail, they'd go back to just women.
I understand that concept, but only that one. I find it offensive for any lesbian to lust after FTMs. Because in that case, they really do abandon the lesbian designation and switch to bi!