Oh, how silly...
By receiving head (an obvious straight-only encounter), Mr. President Bush effectively cancels out any gayness he may have taken on by being topped.
It's basic Closet Math:
1hetero-act + 1homo-act (where 1homo-act = -1hetero-act) = 0
0 being the normative end sum and heterosexuality being the presumed norm, 0 = hetero.
So he can have his sausage if he lets someone else eat it too :-)