That must have been a shock. You have done the right thing separating them. The female likely objected to him trying to mate with her and bit him.
If you left one with nipples in then that will be the female as males do not have nipple lines. How long have you had them? If less than a couple of weeks it is possible one was pregnant when you got them.
I'd check the male today and see how bad the bite is. If small it will likely heal in about 10 days. They can look worse when the blood has spread. If it gets red and sore then he needs to see a vet otherwise it should heal. He will sulk a bit for a while then should be fine.
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