So my DH and I are in a similar situation as other couples here: I have a lover, he doesn't. It all started with hubby's fantasies which in my mind were definitely brought about or at least reinforced by the porn he watched. I wasn't into the idea at all at first, but he was practically obsessed with it so in the end I felt I had to relent. I don't regret it now of course, I'm having the best sex I've ever had and it's refreshing to be able to see someone just for sex, with no strings attached, while still retaining the security of a stable relationship at home.
I think sharing fantasies have always existed, but I agree with others here that the ease of access and amount of content people can find online these days must play a role in escalating kinks. Hubby masturbated to cuckold scenarios (I saw his "secret" browser bookmarks) and it makes sense to me that every time he climaxed to those images, instead of inside me, his kink deepened.
What I will say is that he found reality to be rather different from the fantasy he'd built in his own head. He enjoyed watching me have pleasure, but really struggled with other aspects of the intimacy that took place between me and my lover. The first time he was so traumatised by those things, I said I'd never do it again. But a few days later, to my utter shock, he wanted more. It was obvious to me that he'd masturbated to the memories (and probably more cuckold porn) and somehow his kink had overridden the trauma he'd experienced. So in the end I said to myself - you know what, if that's what he wants, let him have it. He clearly prefers to touch himself in front of porn than to make love to me, or "reclaim" me as other cuckolds do (which I hoped he would but never did).
I won't lie, I was actually annoyed with him and a big part of me starting to see my lover regularly was to get back at my DH for letting his porn addiction totally destroy his desire for a normal physical relationship with me. I felt angry and disappointed.
But we've discussed things at length since that time (it's been two years now) and the situation has normalised. We're honest with each other and clear about our needs. He also knows what he can tolerate, so I usually see my lover at his place. The few times he comes over hubby just listens or watches only briefly, and when he does I tend to go easy on the loving aspects that I know he struggles with. To be clear, I don't "Love" my lover (with a capital L) but as a woman I do need a deeper connection with a man than something purely physical, otherwise I wouldn't function sexually. Maybe other women can, I can't. But hubby understands that, even if he can't bring himself to watch it - yet. No doubt he'll get there eventually, depending what scenarios he masturbates to.