I'll get this out of the way to start with; whether anything is "illegal" or not a 28 year old man dating a 17 year old girl is morally reprehensible, find someone your own age. With that out of the way; I practice in Michigan where this "relationship" would be legal since the age of consent in Michigan is 16. However, if the legal age of consent in Colorado is 18 than my friend you could be charged with statutory rape; a very serious felony that you could assume you would do time for. The fact that this girl may be consenting to whatever it is that is going on is NOT a legal defense. In fact, it is one of a handful of felonies that gives the gift that keeps on giving since upon conviction you would be ordered to register as a sex offender for sizeable portion of the rest of your adult life. Failure to comply with that registry requirement can result in a whole new round of felony convictions. It also leads to regulating where you can live; since convicted pedophiles usually cannot live within a certain distance of schools, playgrounds, parks, etc.
In CO sexual assault means sex between someone 15-17 with someone more than 10 years older, among other things. At the least the 28 year old male could be charged with this crime.