You may have an opportunity to influence that you otherwise wouldn’t. And I don’t just mean influencing those who are adamantly “pro-choice” who may never be swayed (though I know some who have been). I mean those in the group who are silently okay with abortion, who would never speak out in defense of it, but who drove their daughter to the clinic to get hers, who think they are prolife in theory, but in practice, when push meets shove, they choose convenience over life or feel it is okay for others to do so. And those people who are genuinely prolife, but weakly so, who occasionally hear someone in the group defend the prochoice position, but then hear people refute that logic, hopefully with grace and truth. If those who can and do articulate such a position left, the prochoice voice that is now a minority would find deeper and more frequent expression. If this position hasn’t gained a broad presence, then I would be concerned that it could if those with the deepest prolife convictions departed.
If someone in the group believed and taught that black people have no souls, would I agree to be part of a group with them? Well, I would certainly not agree to be publicly associated with them by name. Would I agree to be part of the same group in which I would then be able to publicly and privately disagree with them each time, and try to show them and others the reasons why this position is biblically and morally wrong? Yes.
I guess it does come down to how you view the group. Do you by virtue of the membership endorse the beliefs and choices of fellow members? Clearly there are people at any given time making wrong choices, leaving their spouse without biblical justification, being materialistic, lustful, prideful, etc. But in this case people are actually promoting heresy. I would say if they were being given a voice for that within the group without a sufficient voice to argue against it, that certainly would be sufficient reason for withdrawing.