There are vegans who think it's ok to eat oysters and mussels. I want to go a step farther: vegans should be ok with dairy. Specifically, under pretty conservative assumptions you do much more good donating just $5/year to the Against Malaria Foundation than by avoiding dairy.

How valuable is not eating dairy? The cows don't have good lives. Not as bad as farmed chickens or pigs, but still bad enough that their lives are, all things considered, probably not worth living. By consuming dairy you increase the demand for it, which increases the demand for farmed dairy cattle. If you consume one cow's worth of dairy products, then there will be roughly one more cow in existence to support you.

How big is your lacto-bovine footprint, then? How many cows does it take to supply your dairy desires? Much less than one, actually. Cows are extremely productive, with 6-7 gallons of milk per day on modern farms. Even after accounting for things like cheese being a 10x concentration of milk and 6-7 gallons being only the production during the peak milking years, the typical American's lacto-bovine footprint is just 1/45th of a cow. (Hurford). In other words, if 45 people stop eating dairy then on average there will be one fewer dairy cow suffering. [1]

Let's jump somewhere else: the AMF. GiveWell estimates their "cost per DALY" as being under $100. What's a DALY? It's the cost to give a human an extra year of healthy life. It can mean giving someone an extra year or, giving someone ten years that they would rate as 7/10 instead of 6/10, or anything in between. Now, these weights are kind of a mess and DALY calculations are very sensitive to changes in the initial assumptions, but this number is approximately right. In fact, it's on the pessimistic end, because GiveWell estimates the DALY value without "includ[ing] the many possible benefits of malaria control aside from saving the lives of children under 5". Many people get sick from malaria without dying, and including their suffering would bring the cost/DALY down further.

Now we have a difficult comparison. On one side, keeping a cow from having to spend a year on a dairy farm. On the other side, giving someone an extra year of life. You probably value humans more than cows, but a year on a dairy farm is probably more unpleasant than a human year is enjoyable. I think most people would still lean pretty hard toward the humans here, but let's be conservative and say we think they're equivalent: we would be just as happy for someone to get an extra year of healthy life as we would for a dairy cow not to have to exist for a year.

By avoiding dairy for 45 years you avert one cow year, or by donating $100 to the AMF you give a human an extra year of life. At a 1:1 ratio this means you would achieve similar benefit by giving $100 to the AMF as giving up dairy for 45 years. So you could equivalently give $2/year more and eat typical quantities of dairy or you give $2/less and eat no dairy. Let's pad that to $5 for another round of being conservative: is a year's worth of dairy something you'd rather have than $5?

Some objections and caveats:

This doesn't include the environmental effects of cows.

I see estimates of 17.6 lb CO2e per gallon of milk, 396.4 lb of milk per year, via all dairy products, for the typical American and $2 to offset the emission of a ton of CO2. So a year's worth of dairy needs 50 gallons of milk, which would emit 0.4 tons of CO2e, which would cost $1 to avert. Now, I think $1 to the AMF does much more good than $1 to averting CO2 emissions, so you could raise your minimum donation from $2 to $3 to account for the environmental effect of milk production.

You can build a movement around "no animal products". You can't build a movement around "no animal products except some that aren't that bad".

This is a real concern, but given how much people like dairy it seems pretty rough to say "you could offset the direct harm of your food choice for under $5/year but we think that's too complicated to communicate and so instead dairy is forbidden." Also, this argument seems to hold against the people who think eating honey or bivalves is ok for vegans: these also violate "no animal products" rule and make things more complex.

It's not about complexity, it's about consistency. "We don't exploit animals" is a principle we can rally around while "we don't exploit animals unless we think it's worth it" is slippery.

That's fair, and I think it's one of the strongest objections to my argument here. I agree movements need to be built on consistent principles. I like the principle of "do as much to help others for as little cost to yourself as you can" but if you think most of the best opportunities to help others are in helping animals then "don't enslave animals" is probably a good general rule to push.

You probably wouldn't have posted this if you'd found the offset number was much higher, like $1000/year. Publication bias!

Yes, at $1000/year this isn't a very interesting argument and I wouldn't have bothered with it. On the other hand, the numbers are pretty far below the threshold of interesting; $2/year is much less than, say, $100/year, which would still be interesting.

This doesn't hold for eating meat or eggs; those involve much more suffering so they're harder to offset.

Yes, that's true. This isn't an argument that vegans should eat all animal products, just dairy. On the other hand, beef in particular isn't much worse than dairy; something like 2x worse (Hurford), which would be $4/year.

Shouldn't I be comparing the enjoyment I get out of eating dairy to the suffering the cow goes through to produce the dairy?

When you spend a dollar on yourself do you first ask "is there anyone out there who would get more benefit from this dollar?" If so, that's awesome! If not, though, this is probably because you're willing to value your enjoyment over others to some extent. Valuing yourself infinitely more doesn't make sense, but a simple "do they need it more than me" isn't enough. Instead you should pick a level of sacrifice you're willing to accept and try to do as much to help others as you can within that bound. Which means that if giving up dairy has a better benefit-to-others:cost-to-self ratio than donating to the AMF then you should do that, but if not then you should give to the AMF instead. This post argues that the AMF has a much better benefit-to-others:cost-to-self than avoiding dairy.

I don't eat dairy for health reasons.

That's fine. I'm not arguing (here) that you should change your diet then.

[1] Dairy consumption is somewhat elastic; Hurford estimates this brings the ratio from 1:45 to 1:73. But let's ignore this to be conservative.