2,225. A Prohibition Must Be Unique to Kohanim.

Hilchos Issurei Biah 19:5

If a kohein is intimate with one of the women forbidden as a prohibited relation – excluding a niddah or a woman who is forbidden because of a negative commandment that applies to everyone – he renders her a zonah, as has already been discussed. If he or another kohein is intimate with her again, she is rendered a chalalah and any children she may have with a kohein are chalalim. Therefore, if a kohein is intimate with a woman awaiting yibum and she conceives from their first cohabitation, the child is fit to marry kohanim. The is because the prohibition against intimacy with this woman is not limited to kohanim so she is rendered a zonah, as has been explained. If he’s intimate with her a second time and she conceives, she is rendered a chalalah and the child a chalal because the second intimacy was only prohibited to kohanim.

Hilchos Issurei Biah 19:6

Similarly, if a kohein is intimate with a convert or a freed servant woman, he renders her a chalalah and his children with her are chalalim. If a kohein is intimate with a niddah, the children are halachically valid and not chalalim. This is because the prohibition against intimacy with a niddah applies to everyone and is not unique to kohanim.