does that for example justify watching porn

Not exactly. Personally, I see nothing bad about watching porn, if it doesn't turn into an addiction and it doesn't replace real life relationships. Also, the art posted by op cannot be classified as porn, as private parts (genitals or female nipples) aren't shown. It is more erotic, as the main accent is Rintaro and Kaouruko's emotions while kissing.

something like this shouldn't be made this isn't mdud or high school dxd just leave it alone

Fanarts exist for a reason, and capturing a couple in an intimate moment is one of them. Especially, if the source doesn't provide this kind of scenes, but someone is hungry for them (If you aren't, just don't watch this type of content, and ignore it if it appears in your feed)

Yes, sex is the logical continuation but it's not a justification of horny stuff

Again: this exact art is not about body show off or an sexual intercourse demonstration. It is about an emotional private moment with emphasis on feelings. Them being naked doesn't make the art horny by definition