The way I remember it, Bobby told Katherine "no" because she was Pam's sister and he could never think of Pam's sister as a possible girlfriend. As for April having slept with JR, I liked that Bobby's able to get past that bias. Besides, he was gonna marry Jenna and she slept with (and married) Renaldo Marchetta who is a lot worse than JR. So if women are disqualified from even being considered based on whom they've slept with, if any woman who slept with JR is out of consideration, then a woman who slept with a man worse than JR would then also not be considered. But I like that Bobby's able to become mature and get past something as silly as having to approve of every man a woman has slept with. Is he ready to tell them every woman he's slept with and have them decide if he's slept with any women they don;t approve of? I somehow doubt it. What's more important is that Bobby became more mature and was able to understand that a woman is comprised of a lot of different qualities and to judge her entirely on that one thing and not be able to see there is a lot more to a woman than "who has she slept with." In April's case she had gone though a lot of changes since she was the woman we first saw arrive in Dallas. The woman she was when she was married to Jack is not an accurate indication of the kind of woman she had become since then. When she first arrived in Dallas I didn't like her either.
That's one opinion of April but it's certainly not the only opinion of her and it's certainly not my opinion of her. April became a charming, kind hearted, sexy young lady who was a lot of fun and was someone who would always be loyal to Bobby. She would not walk out in him because she didn;like the way he was running Ewing Oil. Unlike Pam, April enjoyed the oil business and even had an oil company of her own - April Oil. She understood sometimes in business you have to be ruthless. April could understand that the oil business is like a shark tank and you have to become somewhat hard and aggressive to survive in it. After you figure out how you can survive in the oil business, next you have to figure "How can I flourish"? What that entails isn;t always being a nice guy. If you're not the biggest, baddest great white in that shark tank, someone is going to come along and take a big bite out of you. Human beings are multi-faceted creatures. How you have to conduct yourself in order to flourish in the oil business isn't the same way you are going to act when you come home and you're with your wife and kids.
OK, that's fair. I don't know if there are others who might still hate any woman they see as competition to Pam but in your case, you just want Bobby to be happy. I like that