That's cheating I believe. I always look for non-voluntary ntr doujins using cheating tag on both hitomi and nhentai. They work.
The terms "cheating" and "Netorare/netorase" tend to be used in a lot of different ways.
my understanding is that cheating is voluntary only, netorare is when your lover (and it can't be just someone that you like, has to be someone you're actually dating/married to) is stolen away, and netorase is when you willingly let someone else have sex with your lover.
but different places (including this site) handles all that differently. e-hentai (and by extension, ex-hentai) actually use these terms the same way I do. but ryuugames is a different case when it comes to netorase (it's labeled as netorare when it's actually different terms).