Elyon
Yes as long as the MC is involved. Personally I'm not a fan of reading about other characters and it is plain NTR when the MC's partners do it with eachother behind the MC's back. Basically for the second situation imagine some random person coming up to you and talking about their experiences in bed, it is cool that they have it but you don't necessarily want to hear them talk about it.
KopiCAT:So it's NTR when his two lovers do it with each other, but not when he himself does it with a different lover? :/
and for the question in author note, nah yuri when MC isn't included is disgüsting . most people don't like it.. and thinking about it properly ,the harem members are straight ppl falling in love with MC and see him as their lover and no one else , if they have feelings for another person (even if its a girl ) then it make it like an open relationship instead of harem, and MC is like living dildø to them. also imagine in reality in 3some of 2men and 1 woman, will the 2 men start kissing or touching each other body instead of the girl? nah they will not ,because they're normal and straight person . it's the same for girls , don't think everyone entering harem= bullşhit yuri. it only makes the story a wishfulfillment story written by a young author , hope we don't have weird yuri scenes in the harem.