User blog comment:B214/Talk About What You Want! 3/@comment-25825030-20150417222425/@comment-24502975-20150418005550

It's not about being not satisfied with only Rias. He's stated since the very beginning that he wants a harem. Rias has accepted that, and allows him to be with the other girls as well as herself as long as she is the legal wife and, for the moment, his #1 (what I mean by "for the moment" is that eventually he'll be married to all of them and Rias will only be "superior" as the legal wife). Rias wouldn't deny the other girls from their feelings for him, so once she started dating Ise she accepted his harem because she knows she's his #1.

As for cheating... since she accepts it, it's not really cheating. It's polygamy at its finest. She does want to be his first, but if some situation with another girl changes that, it's not like she'll go into rage mode or anything. She'd just demand to be next right then.

You also made a brilliant choice in letting DxD be your first harem-based story. Except now every other story will feel inferior.