The classification of High School DxD as shounen is the official classification as stated by the original japanese publisher, you can verify it on any animes website, there is nothing subjective nor interpreted about it, this is not my personal classification, this is the official classification as stated by the original japanese publisher.
This is an anime about sex and yet nothing sexual ever actually happens even though the protagonist is a pervert surrounded by willing naked girls specifically because it was designed as a shounen meaning for kids and not for adults, hence why it does nothing but tease and blue-ball and why a fan-made adult version to relieve the frustration that it causes is needed, a fan-made adult version would not be needed if the original was designed for adults to begin with, hence here we are.
This is the difference between ecchi which are suggestive shounen and shoujo but don't contain any actual sexual content because they are destined to non-adult demographics and hentai which are actual porn seinen and josei destined to adult demographics.
Here are the official classifications as stated by japanese publishers:
shounen = for boys = non-adult male demographic
shoujo = for girls = non-adult female demographic
seinen = for men = adult male demographic
josei = for women = adult female demographic
Here are further explanations: https://epicstream.com/article/anime-genres-explained-from-shonen-to-seinen-shou...
A relatively reliable way to figure out the demographic target of an anime is their protagonists' demographics as their protagonists usually represent their demographic target.
Indeed, shounen usually center around a boy while shoujo usually center around a girl while seinen usually center around a man and josei usually center around a woman.
Hyoudou Issei, High School DxD's protagonist, is a teenager boy, not an adult man, because High School DxD was designed as a shounen implying that it was destined to teenagers boys, not adults, which is why it is suggestive while not featuring any actual sexual content because it was made for kids and which is why a fan-made adult version is needed to relieve frustration after so much teasing and blue-balling denying what adults want.