Insemination and impregnation role-play are not uncommon for couples in this lifestyle. Some go further, spanning the gap from fantasy to realization, with actual impregnation.
KK and I have kept this in the fantasy domain, but we have 'live played' insemination/impregnation encounters a number of times. Those encounters have included gangbang scenarios, in which the bangers all were led to believe they had the opportunity to impregnate KK anonymously, and encounters with a single, 'select' male chosen to impregnate KK.
We've found both themes equally hot. The participant(s) all believed that they were knocking my wife up, which made the fucking extra-hot, something beyond the routine. Seeing KK with her legs in the air, taking a big cock deep into her pussy, getting pumped full of hot, sticky cum, then positioned with her pelvis rocked back, to retain the stranger's cum is indescribably hot.
Having said all that, we'd never consider actual impregnation and bringing another kid into the mix.