I am a guy but your female friend is right. Having sex is not the right time to discuss lots of things like cuckolding only because everyone is caught up in the moment and the reasoning part of the brain is out the window. You can use cuckolding scenarios to get yourselves going, but that is all you can do. She may just think it is a crazy kink or fantasy you have and leave it at that.
It is best to raise hot wifing, sharing, swinging cuckolding in a non-sexual environment where you can have an intimate discussion without other ears. Ask her what she thinks about it and if there is some interest (ie. not a flat out "are you crazy?"), then you can drill a little deeper and add you own views, interests, desires. A lot of women will be put off when the husband raises it because they will think that somehow you don't love them or find them attractive etc. Which, of course, is the exact opposite. So it may take some digestion time with you constantly expressing your love for her over and over to get her onboard. And even then it might never happen. But you don't know until you try.
My wife and I agreed before we got married that because our jobs entailed travel, that the situation might arise where we wanted to have sex with someone else. We gave each other the green light on the conditions that (a) it would be safe sex and (b) we told the other about it before or just after. No cheating, no lying. We trusted each other and we did each have our sexual trysts and enjoyed them. Then it was an easy road to non-monogamy including cuckolding and even having a boyfriend live with us for a year.