I don't agree with you Prof, the SA are not homophobic & the SA do not proselytise people they help - however we are all free to donate or not to any organisation so fair enough.
It depends on your definition of homophobic - in my world if you systematically as an institution treat individuals less favourably on the basis of their sexuality then you are homophobic - and the SA does.
Regarding proselytising - well there have well publicised issues and do you know what, I wouldn't want to take the risk. Why donate to an organisation where this is a possibility, rather than a secular one where it isn't an issue.
And besides if you want to help the homeless and vulnerable, why donate to an organisation that spends just 22p in every £1 donated on such activities, rather than, say Crisis who spend 62p for every £1 donated on helping vulnerable people.
You are right, we can all make choices, but they should be based on facts about organisations. I suspect many casual donors to the SA would be pretty shocked to find that only 22p in £1 goes on helping vulnerable people, which just about the same amount simply used to run their church, worship and evangelical activities.