If you "doubt Sen. Santorum's conservative credentials," then you know nothing about him whatsoever--which you should, since he's the third-ranker. And not to put too fine a point on it...but Santorum has never said he "supports" homosexuality. And he certainly didn't say it here. If you're saying a gay person should be fired from his job because his sexual preference is discovered (not that this is the case here, as everybody in these circles knew about it), then you may be on the wrong forum altogether.
While I don't support Santorum due to his treachery in last election's primary campaign toward a conservative who was running against Arlen Specter, I don't have any irrefutable argument to put forward that he's not a solid conservative. But, I think it isn't a sure bet that he is. It strikes me that his conservatism is soft and subject to the winds that blow this way and that.
It is entirely likely that he did not know his top aide was a homosexual. Finding that fact out should not be a cause for firing the aide.
However, given that matching one's message is important, there are questions that one would expect to arise if Santorum was entirely cognizant of his top aide's homosexual behavior prior to his being brought on in that capacity. (1) Is it a contradiction to hire someone who is the opposite of your message? (For example, if Ron Paul hired Ralph Nader as his spokesperson, would that raise questions? More to the point: If President Bush appoints Alberto Gonzales to the SCOTUS, does that call into question the Pres's often announced committment to "strict" constructionist judges?) (2) If one finds a disconnect, then does it say that one is dealing with a different definition of conservatism, a different variety of political philosophy albeit similar to conservatism, or perhaps something that is not conservatism at all?
Homosexuality is incompatible with conservatism. That is simple logic based on the definitions of words.