These to me seem like little pimples, not herpes lesions of the buttocks. I can’t be certain of course from picture, but I have seen lots of buttocks herpes and it does not look like this. The thing to remember is that if you have genital herpes you can transmit even if you have no symptoms at all. It is good that you are trying to determine what is a lesion and what isn’t so you can clearly avoid sex when you have obvious outbreak symptoms, that’s one time you can know for certain that you are likely shedding virus.
Remember that you cannot transmit virus through underwear. Remember also that places like your buttocks and thighs and abdomen in the boxer shorts area are not shedding – the skin is too thick there. Virus can’t get out unless there is an outbreak where skin is disrupted; it is only shed from the genitals where the skin is thin.
I am not concerned about these particular pimple like lesions.