I would say that it is possible that it may be moral to unconditionally do X or to unconditionally refuse to do X, yet immoral to do X based on conditions. For instance, it may be moral for a politician to vote against a bill, or to vote for the bill, but it would not be moral to vote for or against the bill based on whether I pay him a bribe. Few people would accept the argument "paying him the bribe doesn't cause him to take any actions that would be immoral in the absence of the bribe".

I would apply that to parents who will only pay for their child's college if the child is straight. Just because they could morally pay (period), or morally refuse to pay (period), doesn't mean that they can morally refuse to pay conditional on the child's sexuality.

And for the Communist analogy to work you would have to say something like "It is moral to pay a charity, and moral to not pay a charity, but immoral to pay a charity conditional on the charity being for a cause you like". which comes out as nonsense.