No, priests should not be allowed to withhold communion. Religion should be about inclusion not exclusion. The self righteous will not inherit the earth.
There are specific laws governing communion. You must not knowingly be in a state of mortal sin. YOU have to be the sinner, not someone who in the future may make that choice. Because you support someone who believes individuals can make their own choice, does not make YOU an occasion of sin. That is sin twice removed and sorry, doesn't qualify. If I was a friend of someone who, in the future, killed someone, I would not be the person committing the mortal sin, the friend would and would not be able to receive communion until he confessed and repented his sin.
Gee, another case of WWJD. Dear Father whatever your name is, please return to 2000 years ago, not a sharp right at the Spanish Inquisition!
Baltimore Catechism, Catholic Encyclopedia, 50 years of Catholic teaching!