With the notive that you have given where you explain "Dragon's age slower than ponies so technically, hes a teenager in dragon years. " You're pretty much telling me I'm right, But more towards Spike being an infant rather a child.
Example (A) Spikes physical appearance. His physical body has the key features of an infantile Dragon. Using the ponies of the show as an example, They have small stubby limbs and short rounded bodies. Thus giving the viewer the clear evidence of them being underdeveloped children (in pony standards.). And this evidence at play, Spike is now proven to be that of an infant Dragon, no matter how long he has lived on that Earth, His physical formality is a child/infant Due to his short stubby limbs and underdeveloped round body. Just look at infantile/child Humans. they give the same physical traits aswell. If Humans and or ponies of Equestria were to have the same slowed aging process that Spike has, They'd to be classified as children/infants.
Example (B) Age gap. Even though Spike does have the physical appearance of a child/infant Dragon, His time on that planet doesn't change anything when it comes to him being in a active or non-active sexual relationship with Rarity. Spike has been alive long enough to learn how to speak and speak in proper sentences aswell as basic tasks because of time+Experience= developed mindset. But that doesn't change the fact that by Dragon standards Spike is still a bay/child. If I recall on the episode where Spike runs off with th other Dragons to find himself, The older more aged Dragons refer to him as a baby/child. Thus the show itself is giving the incentive that Spike "IS" under aged.
Where you say "Plus, if he was a baby, he'd be speaking baby talk and in a diaper. And he has a personality of a teenager.". I can easily out do that note with basic logic, Which was my key point in the above examples. So There so, I am correct and you are not (not to sound childish).
And when it is referred where Spike and Twilight are almost at a 5 year age gap, That really doesn't change Rarity's age gap with spike. Given how old at an estimate Twilight was at Spikes birth/Hatching, We can safely assume she was between the ages of 7-9 at the time comparing to when she was sent to a magic school and to when Humans IRL are sent to there preliminary schools.
Twilight at the current time of the show now would be at an assumption of an age between 18-20, thus spike with his slowed age rate would be maybe with some added math be at the ponied age of 7-10, Give or take. And Rarity given her personality and occupation, She would be at another estimate in her 20's. So the age gap is still at a huge play here given by the laws of natural growth/logic.
If you still refuse these easy to understand facts as to why Sparity Is seemingly an act of pedophilia, Then go ahead and make an idiot of yourself.
Here's some major key logical pointers from a good person I know who thinks the same way.
If you say dragons live longer and Spike is a "Baby Dragon", and by dragon's standards, he's still a kid. But yeah, Spike by ponies' standards, he 's in the teenager's age, but that doesn't change the fact Spike is a kid, not a teenager, according to dragon's standards. Of course he's has a mentality of an adult, but he's still a kid. The thing is that you can't change the age standards from one species to another, for Spike, if he's a dragon, then he lives by dragon's age standards, not by ponies' standards.