Oh, I've confused myself now. I've realised changing to "appear to be" by itself doesn't work either, it needs to be in addition to "being the same sex", eg
"Attraction to people of the same sex and who appear to be that sex"
I'm trying to work through the logic:
Trans women are men by sex category, therefore a relationship with a lesbian woman wouldn't be a same-sex relationship.
Likewise, trans men are women by sex category.
Lesbians are unlikely to be attracted to trans men who pass as men (in the same way they wouldn't be attracted to men).
With me so far?
So by rejecting the TRA line that lesbians should consider trans women as partners, on the grounds that TW are actually men, wouldn't the logical argument be that trans men are actually women and therefore lesbians should consider trans men as partners (as that would result in a same sex relationship)?
Which led me to thinking the definition should include an element of appearing to be a particular sex, in addition to being that sex.
Or maybe we should just reject the notion that anyone "should" consider a particular type of person as a potential partner and leave it to individual choice?
And now I've had another confusing thought. If a natal woman and a trans woman were in a relationship that appeared to the outside world as a same sex relationship, if they received abuse or discrimination based on the perception that they were lesbians, would they miss out on any legal protections because by definition they are heterosexual not lesbian?