Well for starters she negotiated him into the ritual alongside MC in favor of being allowed into the Temple. There no proof ether way that she had sex with Grug before the ritual (not including MC's optional negotiations earlier).
Also she is a priestess of a sex cult, she isnt gonna start fawning over the MC for a couple of assists. He lifted the curse, helped with the Temple, then gets the gemstone. At THIS point she makes him an official member of the cult as gratitude & u can have a 3some with her & Caleah.
It's starting to really bug me & boggle my mind how many people have such hypocrite mindsets on here. "Not gonna f*** these girls cos they been with other dudes", "I hate Ziva because she let's Grug do her".... the MC is screwing like 20 women, without many of them knowing about each other, & spreading his seed across the realm like a mad lad. But no questions the fact we are literally playing a horn dog, wanna-be hero with deceptive tendencies? I am not pleased with some of the scenes but I at least accept them as part of the characters. The relationships evolve from casual flings to actual affection... it's not like theres any true NTR where MC can have girls stolen from him, he dosent have a claim he has to earn it.