This has always caused me confusion. Surely our goal is to survive and reproduce, so why do we hurt so much when another man fucks our women? How did this evolve?

Surely linked to the aggressive male being the most successful, then perhaps wanting all the sex for himself? Still not really sure how it's related. How is that ever beneficial because if it is not beneficial then the trait could not evolve.

The wider context of this question is that I would like to be able to explain to myself and to any cheating girl if that does ever arise exactly what I am feeling and why -- jealousy, betrayal or something else? Where did these labelled emotions come from in relation to infidelity?

Like you'd alluded to, the male brain has evolved to want to keep some sort of monopoly on 'his' woman so as to ensure maximal survival of his offspring (continuation of his genetic likeness).

So how would you describe the feeling? I find it difficult to define. It's not jealousy, it's loss of masculinity, loss of purpose, betrayal.

Subjective. Depends on how you've frame the 'loss'.

Recent research suggests that the same areas of the brain activated during sensations of physical pain are also in operation during emotional pain.

"Loss of masculinity"..."purpose" or "betrayal" aren't feelings. The first two for obvious reasons. Betrayal is an interpretation of what somebody is doing TO you, not a feeling itself. We commonly misuse the word "feeling" (e.g., "I feel betrayed", "I feel alienated") which are actually interpretations of what the other person is doing, real or not. The feeling beneath it may be sad, 'empty', alone, melancholic, unsettled etc..

Because I would rather not raise another mans kid if this girl gets pregnant.

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Fucking valid. +1