I would assume chaffing could be harmful.
other than that I don't think unless you are unfit and suffer a coronary attack you should be ok.... oh and sometimes cramps from unused muscles I guess.
hope this helps out....
only if it is beyond the endurance of body parts and does physical harm is it excessive and unhealthy. I am sure you will know when that happens. Or your partner will let you know. I think the male may know when he is exhausted pretty easily.