@Hotpinkbug,
In most civilized countries, it is illegal for an adult and a 15 year old to have any sexual activity (the law do vary from place to place). When this happens the adult can go to jail. In many places this is considered a serious crime and is called "statutory rape", the idea being that kids don't have ability to make good choices and adults can take advantage of that.
More important is the reason for these laws. I agree with them.
Any good relationship is a relationship of equals. If a 23 year old is dating a 21 or 22 year old, they are both in the same place in life. They both can drive. They both have the responsibility to take care of themselves and to support themselves. They both have the ability to move or to get a real adult job.
If both people in a relationship are adults, then they start as equals. Neither person has more power than the other. They both have the same ability to make their own decisions and take their own responsibility.
If a 15 year old has a relationship with another 15 year old, it the same. They are equals.
An adult dating a teenager is not equal. Think about it. The adult in the relationship has all the money, all the control. He has more experience and knowledge. And, he can use all of these things. It doesn't a fair, balanced healthy relationship. A teenager is hopefully in school. A teenager might have a job, but isn't paying rent or responsible for her own bills. A teenager is making many decisions, but it still being cared for and protected (whether she appreciates it or not).
I understand that real life is complicated. You have these feeling and you know yourself and this guy. There are probably lots of reasons that you think this time, this relationship will somehow work. And, honestly I don't know... maybe it can. But there are an awful lot of ways that this type of unbalanced relationship can go horrible wrong.
I don't support adults having sexual relationships with teenagers.