Why? Just because the members of an organisation believe something that goes against the principles of that organisation, does it mean that the organisation has to change to suit its members?
If it has any interest at all in holding onto them as members, yes.
As HWB rightly reminded us above we're not discussing abortion here, but this is another example of the situation that pertains to both abortion and contraception in Catholicism - most Catholics use some form of artificial birth control, and certainly in the USA, most Catholic women support abortion rights. This means that a majority of members of an organisation or a club (for want of a better word) choose to stay in that club despite strongly disagreeing with some of that club's rules. Same principle at work here; a majority of Anglicans support a form of marriage that their club disagrees with (and tried to prevent even for non-members, as is always the way).
When you have a large proportion of a membership in such stark disagreement with the rules of the club, there are several options. (1) is that they can leave and go nowhere else - Rhiannon has already mentioned this; (2) is that they can leave and join another club whose rules they like better, such as the exodus of disaffected types who converted to Catholicism when the C of E started to ordain women priests back in 1994; (3) is to stay and to try to change the rules of the club from the inside, which always has the potential for (4) the sort of splinter groups and breakaway sects and denominations that religions love which
claim to follow the same rules and
claim to have the same beliefs but actually don't.
The article doesn't make it clear whether the Anglicans surveyed support equal marriage in its civil form - nice of them for sure but neither here nor there as it's a reality and will remain so - or whether their support entails that they want to see same-sex marriages performed in their church. You'd need a more detailed poll for that.