My boyfriend is best friends with a woman he cheated on his last partner with

My boyfriend, A, was in a relationship with E for 5.5 years. A and I got together about a month ago (but have known each other for 7 or 8 years) and as we were discussing dealbreakers, A confessed that he had cheated on E with another women, N, early in the relationship. A says that E was not very physically affectionate and they only had sex 4 times in the entire span of their relationship. A and N (who was/is in a non-monogamous marriage) had been acquaintances prior to the affair, which lasted about 3 weeks. A eventually decided to end the affair, but stayed with E for 4 more years, during which she never found out that he cheated, and during which A and N (after some months of awkwardness) became best friends. Eventually A and E broke up. A started going to therapy and worked through the issues that had caused him to cheat, all the while nurturing a friendship with N. I am the first person he has been in a relationship with since the affair and he voluntarily told me about his prior infidelity with N and that they were just close friends and he looked at her more as a sister now. He hopes that I will be accepting of their friendship. I am VERY uncomfortable with this despite the fact that it's been 6 years and that he is a different person now and no longer has romantic or sexual feelings towards her. There's just something about my boyfriend asking me to be cool with his friendship with his former mistress that just rubs me the wrong way. He betrayed his former partner with this woman and wants me to be okay with her being a major part of his life? Am I wrong for feeling this way? Aside from this situation, he is everything I ever wanted. Emotionally available, validating, supportive, etc. I don't want to end the relationship but I also don't want to be uncomfortable. I feel like the only way for everyone to win is if I meet her and genuinely like her and want to be her friend also. I really don't think there's a good chance of that though. I don't know how we can move forward like this.