Difference in age diminishes as age increases, due to the lower percentage impact.
The difference between a 13 year old and a 15 year old is significant, 17 and 15 less so, 17 and 19 less so, etc, etc, etc.
I don't see anything wrong with 15 & 17. As far as the whole stat. rape thing, I believe most states have a provision for consensual sex between people of close age, but one being below the legal age of consent. For example, an 18 year old with a 17 year old g/f. Check your state laws if you're worried about that, or (my suggestion) simply don't have sex.
Viper GTS