I agree with you here. Unfortunately this type of attacking sometimes works, although thankfully seems to not be this time around (at least in FL). I wasn't really saying religion defines who he is, but rather in responding to Communion about why, although Mormon ideals in black and white seem a bit offputting, they don't "put me off" of Romney's campaign.
Attack ads and the moronic political landscape aside, it still is perfectly normal to identify with someone based off of their personal life, and it certainly influences the voter base, although we are in agreement that in the scenario you laid out here it's perfectly stupid to stereotype the candidate thusly iand base your opinion of him off of attack stereotypes in totality.