..., but their logical arguement can not simply be -
"Homosexuality is immoral because the Bible sez so. QED."
I don't see why not. You generally cannot derive moral principles using logic. A person might just say "homosexuality is immoral" and not even give a reason.
Generally speaking there are social pressures that seem to expect that people give a reason. And if you pressure people for a reason, most will give one. But if their particular moral position is not based on reason, then the reason they will give you may be entirely bogus. It is simply the rationalizing of a decision that was not made on rational grounds.
Morality can be determined by a philisophical stand.
This presumes people have thought through things enough to have developed a philosophical stand. And why can't a philosophical stand be an eclectic mixture that includes such basic principles as "homosexuality is immoral"?