If the herpes virus is limited to the genital area, then in theory it will not be passed on during oral sex, so the condom would not be necessary.
The difficulty is that a person could have a disease without symptoms, eg a cold sore is contagious in its early stages before a blister appears. Cold sores are HSV 1 and are present in about 70% of population, these could cause HSV1 blisters in any area including genital.
So the chances of genital herpes HSV2 being passed on to you via oral sex is slim, but there are other lesions which could occur.
So a condom will offer protection from HSV1.
We try to play safe with advice, in order to reduce the amount of these viruses being passed on, but at the end of the day you need to consider whether the risk is important to you.
Regards
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