I voted giving up meat.
I don't get the point of the discussion though or how this came up among you and your group of friends.
I guess both are 'lifestyle' choices in a sense.
I'm not a vegan but I know a few and they seem happy, healthy, etc.
I was virgin until I got married but I know people who are happy, healthy, etc. with abstinence, lots of sex, etc.
Both 'lifestyles' are viable for a human being (ignoring the whole procreation thing for right now) so ... oh wait... i get it.
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The point of the discussion and how this came up among you and your group of friends is simply because the topic is on sex. Must be a mixed group (male and females) and can be used as a 'ice-breaker'/topic maker since most people will laugh/joke about it and be able to make flirting remarks with the opposite gender. Regardless of whether the girl or guy picks sex or meat, you get to know the person and maybe joke with them about it and make it into something more.
Example: Guy picks meat (to keep, give up sex). Girl who likes him says, "oh, too bad." or something to the effect that she could make him give up meat. Something similar if girl picks meat (give up sex), guy says something like you just haven't had "the right partner". If they pick sex (give up meat), then could say 'dang, i really want to see what you got going on' or various other flirting lines.
If, for some strange reason, this topic was mentioned in an all guy group... it would be awkward. People would vote and then the topic would 'die' since there isn't much else to talk about. All girls could get some more mileage out of the topic but only a mixed group with a slant towards sexual activity would come up with this topic.
If, for some reason, this topic came up and no one meant for it too... uhh... strange and I'd call BS.
Corvuus