Faye, The Reason that Mormons don't drink alcohol basically boils down to that the Lord revealed to his children that we should abstain from certian things. Here's what was said by the Lord about that. " 5. That inasmuch as any man adrinketh wine or strong drink among you, behold it is not good, neither meet in the sight of your Father, only in assembling yourselves together to offer up your sacraments before him.  
6. And, behold, this should be wine, yea, apure wine of the grape of the vine, of your own make.  
7 And, again, astrong drinks are not for the belly, but for the washing of your bodies.  
8 And again, tobacco is not for the abody, neither for the belly, and is not good for man, but is an herb for bruises and all sick cattle, to be used with judgment and skill." Now of course my response begs other questions, like "Where is that found at in the scriptures?" But to keep the post on topic, I won't go into that. I guess one reason that they have chosen not to serve alcohol at the reception/wedding is they feel that they would be enabling others to do things that they see the Lord has frowned upon. As far as being in the company of others who do drink/smoke, that depends on the person. I personally know that everyone can make thier own decisions, and I should love everyone unconditionally. Unfortunately, there are those, (just as in every religion) who aren't as tolerant, and who you would feel shunned by if you were to smoke/drink around them. But I'm sure that your son (hopefully) understands where you are coming from, and will be one who is tolerant. 
 
Oh, and "No" Drinking isn't in the same category as beating your children. Beating your children is much worse.