Bruh, to anyone saying that "a 15 year old is an adult" I beg you to please do even surface research on how human brains develop.
The part of our brain that handles logical thought and calculates the possible repercussions of our actions isn't fully developed until we're in our mid 20s.
If we're talking realistically, humans shouldn't even be considered adult at 18, which is the most common age of majority in the world.
Also, we're pre-determined towards harmful behavior between ages ~15 and ~25, meaning that not only do we not have brains that are fully developed in regards to calculating the consequences of our actions, but we're also going to be more likely to put ourselves into risky situations. It's a dangerous combo, and it's part of the reason we have laws to protect those in that situation, even though those protections really should continue for a few more years past where they currently end.
Also, a 15 y/o having sex with another 15 y/o is way different than a 15 y/o having sex with someone in their 20s+. The person in their 20s+ has way more life experience and is further along in their brain development, which causes a clear power imbalance that can be exploited by the older person to manipulate the younger one. We don't live in a void where age is just a number.
Once both parties are at least mostly mature and have had the chance to gain relationship/life experience, then the age gap doesn't really mean much because both parties can fully calculate the risks and rewards of the relationship. Personally, I believe the age that someone should be considered 100% an adult should be 23 at the lowest, but that number can vary from person to person. However, it's never going to be as low as 15, and even 18 is too low if you look at all the facts.
Like, you can still enjoy trashy manga like this if you are able to fully recognize that it's fiction and that this would under no circumstances be acceptable in the real world. I find it funny that the people who will always jump to saying "it's just fiction" are the same ones that try to justify the situations in said fiction.