Meat on Friday is a peripheral issue because it does not touch the issue of life and death.
In this sense, a Catholic President cannot “impose” (for want of a better word) such a rule on others.
Abortion on the other hand is different. It transcends religion and is a constitutional one. In this case, you HAVE TO IMPOSE some sort of morality on someone.
You HAVE TO follow some sort of morality whether it is informed by your religion or not.
If a Catholic President chooses not to use birth control because of the dictates of his religion - good for him. If a Catholic President feels it is his place to lecture us on the evils of contraception - nuts to him, most people don't want to hear it. If a Catholic President feels there is a compelling government interest in regulating or forbidding contraception because his religion teaches it is wrong - that is not consistent with conservatism and the idea of a government of limited and enumerated powers where the citizen is sovereign and we all have freedom of conscience.