User blog comment:Animegeek1689/Does Issei really love Rias?/@comment-23853100-20180504100138/@comment-28884330-20180808215344

Because a Harem King does not have a "main girl". Issei said he would make everyone who wants to be with him happy, not that they would be after-thoughts when compared to a single one. That is not a harem king, that is having a wife and side-dishes.

Rias getting Issei's first time is fine, but it's not some right she has or something she should be promised, because favoritism is directly opposed to the oath Issei took. For a harem king, every girl who loves him and he loves are "main girls".