I understand what "harmful on net" means, and I'm not sure why you think I don't. The point is that there are different things that might or might not be "harmful on net", and you need to not mix them up, and I think you are mixing them up. Specifically, "is drinking alcohol harmful on net?" and "is being allowed to drink alcohol harmful on net?" are very different questions, because of the things I listed that are functions of whether people are allowed to drink alcohol more than of whether they actually do.

The first part of that doesn't work by itself, since Lewis believes in compulsion for, for instance, anti-murder laws.

I'm afraid I don't understand what argument you're making. It appears to have the form "Such-and-such a proposition about alcohol prohibition is wrong, because C S Lewis believed in compelling people not to commit murder" and I don't even understand how anything of that form could be right -- because there are potentially relevant differences between drinking alcohol and committing murder. (Examples: most people who disapprove of drinking alcohol think that murder is much, much worse; empirical evidence suggests that prohibiting alcohol is liable to result in a very large black market in alcohol, while prohibiting murder results in only a small black market in murder.)

[EDITED to fix a trivial typo in the foregoing paragraph.]

if you became convinced that the side effects of prohibition weren't as bad as you originally believed, you would then support prohibition.

Yes, or at least almost. (Well, not me because as I said above I wasn't describing my own position on alcohol. But someone who holds that position would indeed switch to approving of prohibition if they decided that the side effects of prohibition and the badness of the compulsion itself didn't outweigh the harm done by drinking. The bit in italics is why I say "almost" rather than an unqualified "yes".)

The question then becomes "would Lewis think there are really bad side effects to not allowing same-sex marriage".

I don't know what Lewis would have said about same-sex marriage if the question had been put to him in such a way as to get it taken seriously despite his society's general presumption against the idea. For what it's worth, I think he probably would have opposed same-sex marriage (perhaps arguing that it is simply impossible for two people of the same sex to marry, and that calling anything a same-sex marriage is an abuse of language), but if not then it would probably have been on grounds of freedom rather than of bad side effects of prohibition. (You can prohibit certain classes of marriage without needing much intrusion into individuals' lives; it's hard to see how there'd be scope for a big black market in same-sex marriages; any precedents established by the prohibition would probably also be ones Lewis would have been inclined to approve of.)

It may be worth noting that I am not Richard Kennaway and am not necessarily arguing for the same position as he is.