Contrary to popular belief that's not really so much a thing. Even back in the day RP was only vaguely popular with a portion of the community. Most furries don't have sex in fursuit, only some.

Otherwise it's not yiffing, it's just sex. Probably more prevalent among furries given that we have a higher ratio of queer people than wider society, and we're all mostly more open about sex. Straight non-furs are comparatively Catholic in their relations.