Is it important to be vegetarian when meditating "Metta meditation"?

The Buddha clearly and explicitly prohibited vegetarianism for the monastics. This was part of the controversy that culminated in the heretic Devadatta's failed attempt to foment a schism and to murder the Buddha.

There are various possible reasons for this, all speculative: the obligation of monastics to accept alms to enable ordinary people (puthujjana) to acquire merit, many of whom would not have been able to offer non-meat items to the monastics, seems to be the most likely reason; the scarcity of non-meat food items in many areas, which would cause monastics to leave those areas or starve (starvation was a constant problem for the monastics); religious pride and self-righteousness; and the law of karma, which only holds an individual responsible for actions of which he or she is the direct cause.

The Buddha did forbid that any animal should be killed specifically for a monastic, and monastics were not allowed to accept such offerings. Some Mahayana sutras prescribe vegetarianism, however.

One part of your bodily sensations are due to food we take. Light vegetarian food can help concentration and meditation including Metta. This is beneficial but not necessary.

Buddhism is not an extreme ideology and centres around intention. If you do not have the intention of killing and also the situation is not such what you would be tainted by the guilt that an animal was killed on by behalf, then there is nothing wrong to eat non Veg. food. The main thing is keeping your mind pure and conducive to meditate hence if you find yourself uneasy or guilty about eating non Veg. food then it will be beneficial to give up.

But if you are a monk you have no choice but accept what has been offered.

When my SO became vegetarian she said she liked animals (e.g. horses and cows and so on) and other living creatures; and said it would be hypocritical to cause them (e.g. to pay a butcher for them) to be killed for her to eat.

I think she felt that being non-vegetarian (i.e. that being a carnivore or predator) would interfere with or cloud her perception of her love for living animals.

I think there's some justification for that in a text like the Dhammapada, for example in the whole chapter on violence, which starts for example with,

All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.

I think that, logically, that justification depends on three assumptions:

• That the "all" (who tremble at violence) includes annimals, i.e. sentient beings
• That lay people who buy their food encourage or cause killing if they pay for meat (that argument is slightly different or doesn't apply to monks who take alms and who eat whatever they are given)
• That vegetarianism is feasible (practicable) and is not contrary to the doctrine of the middle way

Vegetarianism is sila or part of the training in virtue. According to Buddhaghosa, such moral discipline serves two functions - to compose the mind and to aid in the cultivation of non-regret. I'd say it's fairly obvious that if you are on the cushion and worrying about what you take to be a contradiction between your actions and the lovingkindness meditation you are practicing, then yes, you might benefit from vegetarianism.

It's important to understand, however, that eating meat isn't going to "short circuit" your metta meditation. If that were the case, than the whole of Tibetan Buddhism would be marked by deficiencies in lovingkindness! I will say, however, that eating a vegetarian diet does amazing things for your concentration and that these benefits extend well beyond the absence of regret. Like anything else, the proof of the pudding is in the eating, right? Just not a meat pudding in this case. ;-)