Interjection: No. They are because it is Assur to have any pleasure from any interaction with a woman who is forbidden to you (including a Niddah).
And yes, technically the Avos could violate Halachah before Mattan Torah. But we assume that they didn’t unless we are forced to say otherwise. And the Shvatim marrying their sisters isn’t explicit in the P’sukim and is also probably a Machlokes Hamidrashim (and also a bit against the P’sukim that tell us that Y’hudah married the Adulami’s daughter.)