To me honestly lust is just lying about sex. Being poly when a partner tells me they had somebody over to fuck while I’ve been on my 3 day work stretch I’m like high five buddy get it but finding a dating app on their phone I didn’t know about or intimate texts I didn’t know about or that they cut into my time with them and told me they were sick or working when they were with another person are all not ok. Same thing when I’ve made relationship fuckups it was never really the having sex with someone else specifically it was that I was doing that after saying I was willing to be monogamous and not fessing up when I realized I couldn’t be.