Not remotely, it's just plain old cheating. The mother is participating in the affair of her own (quite enthusiastic, even!) free will, whereas the affair beginning with rape or some form of coercion or trickery is an essential element of the fetish that this site's very incorrect tag definition fails to take into account.
Tentacular, now that is Jackerman doing NTR. And the good kind, where the focus is on the primary victim (the one being raped/coerced, where the thrill comes from seeing them find it more enjoyable than sex with their partner and experiencing guilt, shame, and denial over that until ultimately embracing it) rather than the secondary victim (the one being cuckolded, where the... "thrill"... is supposed to come from vicariously experiencing their feelings of loss and betrayal).