I have a cut on my finger.

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What if I got cum on my finger, could that be bad? Do I need to worry?

i don't think so

why would that be bad? do you plan on fingering a girl immediately after?

You have a "cut" and are worried about "cum" getting in/on it?

If it's your own cum, no, nothing to worry about at all. If it's someone else's and they had an STD, then just conceivably it could be spread that way. A band-aid would be a reasonable precaution, unless it were someone with AIDS, in which case you'd want to be more careful.