Pre the current transactivist movement, my experience (and that of many other lesbians) is that we received this pressure from outside the LGB movement from people who were (and called themselves) heterosexual men (and also sometimes from family/community).
E.g. Straight men deliberately targeting you because you were a lesbian and that was a challenge, telling you you were a "man hater" (ie bigoted) because you wouldn't have sex with them, being told that you just hadn't met the right man yet and needed to give it a try, conversion therapies (one woman I knew was sent for therapy to cure her, another one - from a non-UK background - was exorcised), I have been targeted for sexual harassment because I am a lesbian, some lesbians are subjected to corrective rape etc
When we entered the LGB community we were told that this was wrong. that there was nothing wrong with being a lesbian and that we were entitled to our boundaries.
Now, since the trans umbrella expanded, in particular to include heterosexual cross dressers, the pressure is coming from within our own community - For us, opinions like JAPAB's aren't just something you very occasionally see on Mumsnet - It is a constant and from within our own community and the suspicion of being 'non-inclusive' (ie not having sex with people with penises) is from much on lesbians specifically.
@JAPAB - Most of the GBT/"progressive" people who think lesbians need to be re-educated to overcome our "prejudice" about having sex with people with penises seem to have started doing so since the current transactivist movement took hold focusing on the belief that lesbians should have sex with people with penises who identify as trans. However, from what you are saying, is it broader than that? Do you think we were also wrong before that ie to have been "prejudiced" against having sex with (non-trans-identified) men?
When there are four groups potentially affected by this (straight men, gay men, straight women, lesbians) why do you think it is specifically the fourth group that is getting the vast majority of the pressure on this?